diff --git a/frontend/src/lib/components/account/EditRequestModal.svelte b/frontend/src/lib/components/account/EditRequestModal.svelte
index 20492f0..33cb05e 100644
--- a/frontend/src/lib/components/account/EditRequestModal.svelte
+++ b/frontend/src/lib/components/account/EditRequestModal.svelte
@@ -502,8 +502,16 @@
 			const endStr = `${endYear}-${String(endMonth).padStart(2, '0')}-${String(daysInEndMonth).padStart(2, '0')}`;
 
 			const [whRes, ahRes] = await Promise.all([
-				fetch(`/api/scheduling/working-hours?start=${startStr}&end=${endStr}`),
-				fetch(`/api/scheduling/available-hours?start=${startStr}&end=${endStr}`)
+				fetch(`/api/scheduling/working-hours?start=${startStr}&end=${endStr}`, {
+					headers: authStore.currentToken
+						? { Authorization: `Bearer ${authStore.currentToken}` }
+						: {}
+				}),
+				fetch(`/api/scheduling/available-hours?start=${startStr}&end=${endStr}`, {
+					headers: authStore.currentToken
+						? { Authorization: `Bearer ${authStore.currentToken}` }
+						: {}
+				})
 			]);
 
 			if (whRes.ok && ahRes.ok) {
@@ -570,8 +578,16 @@
 			const endStr = endOfMonth.toString();
 
 			const [whRes, ahRes] = await Promise.all([
-				fetch(`/api/scheduling/working-hours?start=${startStr}&end=${endStr}`),
-				fetch(`/api/scheduling/available-hours?start=${startStr}&end=${endStr}`)
+				fetch(`/api/scheduling/working-hours?start=${startStr}&end=${endStr}`, {
+					headers: authStore.currentToken
+						? { Authorization: `Bearer ${authStore.currentToken}` }
+						: {}
+				}),
+				fetch(`/api/scheduling/available-hours?start=${startStr}&end=${endStr}`, {
+					headers: authStore.currentToken
+						? { Authorization: `Bearer ${authStore.currentToken}` }
+						: {}
+				})
 			]);
 
 			if (whRes.ok && ahRes.ok) {
diff --git a/frontend/src/lib/components/admin/WalkInBooking.svelte b/frontend/src/lib/components/admin/WalkInBooking.svelte
index edb9921..c46d127 100644
--- a/frontend/src/lib/components/admin/WalkInBooking.svelte
+++ b/frontend/src/lib/components/admin/WalkInBooking.svelte
@@ -4,7 +4,7 @@
 	import { authStore } from '$lib/stores/auth.svelte';
 	import { CalendarDate } from '@internationalized/date';
 	import { SvelteDate } from 'svelte/reactivity';
-	import { onMount } from 'svelte';
+	import { onMount, onDestroy } from 'svelte';
 	import { toast } from 'svelte-sonner';
 
 	import type { AvailableHoursDay, Service } from '$lib/types/booking';
@@ -66,6 +66,43 @@
 		return () => clearInterval(interval);
 	});
 
+	// Release any held reservation if the component unmounts while a slot
+	// is still reserved (e.g. admin navigates away from the page). Best-effort
+	// — TTL cleanup will eventually run if this fails.
+	onDestroy(() => {
+		if (typeof window !== 'undefined' && window.__walkInCountdownInterval) {
+			clearInterval(window.__walkInCountdownInterval);
+		}
+		if (_reservationId) {
+			releaseWalkInReservation();
+		}
+	});
+
+	async function releaseWalkInReservation() {
+		if (!_reservationId) return;
+		const idToRelease = _reservationId;
+		// Clear local state first so a slow DELETE doesn't block the UI.
+		_reservationId = null;
+		reservationExpiresAt = null;
+		_reservationCountdown = '';
+		reservedDuration = 0;
+		reservedStartTime = null;
+		if (window.__walkInCountdownInterval) {
+			clearInterval(window.__walkInCountdownInterval);
+		}
+		try {
+			const res = await fetch('/api/admin/bookings/reserve', {
+				method: 'DELETE',
+				headers: { Authorization: `Bearer ${authStore.currentToken}` }
+			});
+			if (!res.ok && res.status !== 404) {
+				console.warn('Failed to release walk-in reservation', idToRelease, res.status);
+			}
+		} catch (e) {
+			console.warn('Error releasing walk-in reservation', idToRelease, e);
+		}
+	}
+
 	function timeToMinutes(time: string): number {
 		const parts = time.split(':').map(Number);
 		return parts[0] * 60 + parts[1];
@@ -375,14 +412,7 @@
 
 	function handleModalClose() {
 		showCreateModal = false;
-		_reservationId = null;
-		reservationExpiresAt = null;
-		_reservationCountdown = '';
-		reservedDuration = 0;
-		reservedStartTime = null;
-		if (window.__walkInCountdownInterval) {
-			clearInterval(window.__walkInCountdownInterval);
-		}
+		releaseWalkInReservation();
 	}
 </script>
 
diff --git a/frontend/src/lib/components/booking/BookingFlow.svelte b/frontend/src/lib/components/booking/BookingFlow.svelte
index 03edca9..a94f2da 100644
--- a/frontend/src/lib/components/booking/BookingFlow.svelte
+++ b/frontend/src/lib/components/booking/BookingFlow.svelte
@@ -40,6 +40,7 @@
 	import UserPaymentModal from '$lib/components/payments/UserPaymentModal.svelte';
 	import { extractBookedSlots, getLunchProtectionForSlots } from '$lib/lunchProtection';
 	import { formatLocalDateTime, getLondonTodayCalendarDate } from '$lib/utils/timeSlots';
+	import { generateUUID } from '$lib/utils/uuid';
 
 	import type {
 		Service,
@@ -88,6 +89,11 @@
 	let selectedPaymentMethod = $state<string | null>(null);
 	let showNewCardForm = $state(false);
 	let isProcessingPayment = $state(false);
+	// Synchronous double-click guard. Svelte 5 reactivity is async (effects run
+	// on the next microtask), so `isProcessingPayment` may not propagate to the
+	// button's `disabled` binding before a fast second click fires. This non-
+	// reactive flag is checked synchronously at the start of processPayment.
+	let isProcessingPaymentSync = false;
 
 	// New card form fields
 	let newCardNumber = $state('');
@@ -282,6 +288,11 @@
 	}
 
 	async function processPayment(amount: number) {
+		// Synchronous double-click guard — set BEFORE any await so a rapid second
+		// click is rejected immediately, even before the reactive `disabled` has
+		// propagated to the button.
+		if (isProcessingPaymentSync) return;
+		isProcessingPaymentSync = true;
 		isProcessingPayment = true;
 		paymentAttempted = false;
 		try {
@@ -296,7 +307,7 @@
 			const body: Record<string, unknown> = {
 				payment_type: 'deposit',
 				amount: amountCents,
-				idempotency_key: crypto.randomUUID?.() ?? Date.now().toString()
+				idempotency_key: generateUUID()
 			};
 
 			if (selectedPaymentMethod) {
@@ -321,9 +332,16 @@
 
 			if (response.ok) {
 				depositPaid = true;
-				confirmedBooking.deposit_paid = true;
-				confirmedBooking.amount_paid = (confirmedBooking.amount_paid || 0) + amount;
-				confirmedBooking.amount_due = Math.max(0, (confirmedBooking.amount_due || 0) - amount);
+				// Immutable update — avoid mutating the existing object so
+				// concurrent renders (e.g. a stale fetch) can't observe partial
+				// state. (See audit: HIGH issue #3 — confirmedBooking mutated
+				// in place, potential overcharge on double-click race.)
+				confirmedBooking = {
+					...confirmedBooking,
+					deposit_paid: true,
+					amount_paid: (confirmedBooking.amount_paid || 0) + amount,
+					amount_due: Math.max(0, (confirmedBooking.amount_due || 0) - amount)
+				};
 				toast.success('Payment successful!');
 			} else {
 				const text = await response.text();
@@ -335,6 +353,7 @@
 			);
 		} finally {
 			isProcessingPayment = false;
+			isProcessingPaymentSync = false;
 		}
 	}
 
@@ -416,6 +435,34 @@
 		}
 	}
 
+	// Best-effort release of the currently held reservation. Idempotent: safe
+	// to call when no reservation exists. Clears all reservation state so a
+	// subsequent reservation must be re-acquired.
+	async function releaseReservation() {
+		if (!_reservationId) return;
+		const idToRelease = _reservationId;
+		// Clear local state first so a slow DELETE doesn't block the UI.
+		_reservationId = null;
+		reservationExpiresAt = null;
+		reservationCountdown = '';
+		reservationExpired = false;
+		_reservedSlotTime = null;
+		_reservedSlotDate = null;
+		if (window.__bookingFlowCountdownInterval) {
+			clearInterval(window.__bookingFlowCountdownInterval);
+			window.__bookingFlowCountdownInterval = null;
+		}
+		try {
+			const res = await fetch('/api/bookings/reserve', {
+				method: 'DELETE',
+				headers: authStore.currentToken ? { Authorization: `Bearer ${authStore.currentToken}` } : {}
+			});
+			if (!res.ok) console.warn('Failed to release reservation', idToRelease, res.status);
+		} catch (e) {
+			console.warn('Error releasing reservation', idToRelease, e);
+		}
+	}
+
 	function startCountdown() {
 		if (!reservationExpiresAt) return;
 
@@ -1153,6 +1200,12 @@
 			selectedServices = [...selectedServices, service];
 		}
 
+		// Service change invalidates any held reservation (duration may have changed).
+		// Release before re-fetching hours so the next reservation starts fresh.
+		if (currentStep >= 2 && _reservationId) {
+			releaseReservation();
+		}
+
 		// Only clear if we're on the date/time selection step
 		if (currentStep === 2 && selectedDate) {
 			const monthKey = `${selectedDate.year}-${String(selectedDate.month).padStart(2, '0')}`;
@@ -1193,14 +1246,30 @@
 
 	// Select a time slot with server-side re-validation
 	async function selectTimeWithValidation(time: string) {
+		// Race-condition guard: capture the user's click and pass it explicitly
+		// to the validator. If the user clicks a different time before this
+		// validation completes, the stale result must not clobber the newer
+		// selection.
 		selectedTime = time;
-		await refreshAndValidateSlot();
+
+		// Time change invalidates any held reservation (slot is now stale).
+		// Only release if the new time differs from the reserved one — if the
+		// user is re-clicking the same reserved time, keep the reservation.
+		if (_reservationId && time !== _reservedSlotTime) {
+			await releaseReservation();
+		}
+
+		await refreshAndValidateSlot(time);
 	}
 
-	// Re-fetch available hours silently and check if selectedTime is still available
+	// Re-fetch available hours silently and check if the requested time is still
+	// available. `validateForTime` should be the time the user clicked, NOT the
+	// current selectedTime — this prevents a slow validation for a stale click
+	// from clearing a newer selection.
 	// Uses skipLoadingFlags=true to prevent UI judder (loading spinners hide DatePicker/TimeSlotPicker)
-	async function refreshAndValidateSlot() {
-		if (!selectedDate || !selectedTime) return;
+	async function refreshAndValidateSlot(validateForTime: string | null = null) {
+		const timeToCheck = validateForTime ?? selectedTime;
+		if (!selectedDate || !timeToCheck) return false;
 
 		const monthKey = `${selectedDate.year}-${String(selectedDate.month).padStart(2, '0')}`;
 		delete availableHoursCache[monthKey];
@@ -1210,12 +1279,12 @@
 		const dayAvailable = availableHours?.[dateStr]?.slots;
 		if (!dayAvailable || dayAvailable.length === 0) {
 			toast.error('Sorry, this slot is no longer available. Please choose a different time.');
-			selectedTime = null;
+			if (selectedTime === timeToCheck) selectedTime = null;
 			return false;
 		}
 
 		const duration = getTotalDuration();
-		const [selHour, selMinute] = selectedTime.split(':').map(Number);
+		const [selHour, selMinute] = timeToCheck.split(':').map(Number);
 		const selStart = selHour * 60 + selMinute;
 		const selEnd = selStart + duration;
 
@@ -1227,7 +1296,7 @@
 
 		if (!stillAvailable) {
 			toast.error('Sorry, this slot was just taken. Please choose a different time.');
-			selectedTime = null;
+			if (selectedTime === timeToCheck) selectedTime = null;
 			return false;
 		}
 
@@ -1317,29 +1386,7 @@
 				currentStep = 3;
 			} else {
 				// Different slot or no reservation — release old one first, then reserve new
-				if (_reservationId) {
-					try {
-						const res = await fetch('/api/bookings/reserve', {
-							method: 'DELETE',
-							headers: authStore.currentToken
-								? { Authorization: `Bearer ${authStore.currentToken}` }
-								: {}
-						});
-						if (!res.ok) console.warn('Failed to release old reservation', res.status);
-					} catch (e) {
-						console.warn('Error releasing old reservation', e);
-					}
-					_reservationId = null;
-					reservationExpiresAt = null;
-					reservationCountdown = '';
-					reservationExpired = false;
-					_reservedSlotTime = null;
-					_reservedSlotDate = null;
-					if (window.__bookingFlowCountdownInterval) {
-						clearInterval(window.__bookingFlowCountdownInterval);
-						window.__bookingFlowCountdownInterval = null;
-					}
-				}
+				await releaseReservation();
 
 				const slotStillFree = await refreshAndValidateSlot();
 				if (!slotStillFree) return;
@@ -1379,24 +1426,12 @@
 	async function submitAndProceed() {
 		_isSubmitting = true;
 		try {
-			if (!idempotencyKey) {
-				// Generate UUID v4 manually for environments where crypto.randomUUID() is unavailable
-				const array = new Uint8Array(16);
-				if (typeof window !== 'undefined' && window.crypto) {
-					window.crypto.getRandomValues(array);
-				} else {
-					for (let i = 0; i < 16; i++) array[i] = Math.floor(Math.random() * 256);
-				}
-				array[6] = (array[6] & 0x0f) | 0x40; // version 4
-				array[8] = (array[8] & 0x3f) | 0x80; // variant 1
-				idempotencyKey = [...array]
-					.map((b, i) => {
-						const hex = b.toString(16).padStart(2, '0');
-						if (i === 4 || i === 6 || i === 8 || i === 10) return '-' + hex;
-						return hex;
-					})
-					.join('');
-			}
+			// Always generate a fresh idempotency key per submission attempt. The
+			// previous behavior kept the key across the component lifetime, which
+			// meant a second submission (after a success, back-nav, change) would
+			// hit the backend with the same key and get the ORIGINAL booking back,
+			// making the user believe they made a new booking when they didn't.
+			idempotencyKey = generateUUID();
 
 			if (!selectedDate || !selectedTime) {
 				toast.error('Please select a date and time');
@@ -1479,6 +1514,12 @@
 					total_amount: booking.total_amount || getTotalPrice(),
 					duration_minutes: booking.duration_minutes || getTotalDuration()
 				};
+				// Booking is now persisted — release the temporary reservation
+				// (best-effort, no UI block). Also clear the idempotency key so
+				// any future submission in this component lifetime gets a fresh
+				// key and is treated as a NEW booking attempt.
+				releaseReservation();
+				idempotencyKey = '';
 				currentStep = finalStep;
 				fetchDiscountPreview();
 				setTimeout(() => {
@@ -1531,6 +1572,12 @@
 
 	function prevStep() {
 		if (currentStep > 1) {
+			// If leaving step 2 (date/time) and a reservation is still held, release
+			// it — the user is abandoning this selection. Going from step 3 back to
+			// step 2 keeps the reservation (the user is just reviewing).
+			if (currentStep === 2 && _reservationId) {
+				releaseReservation();
+			}
 			currentStep--;
 			setTimeout(() => {
 				window.scrollTo({ top: 0, behavior: 'smooth' });
@@ -1764,6 +1811,10 @@
 									maxValue={maxCalendarDate}
 									{isDateUnavailable}
 									onchange={(newDate) => {
+										// Date change invalidates any held reservation (slot is now stale).
+										if (_reservationId) {
+											releaseReservation();
+										}
 										selectedDate = newDate;
 										selectedTime = null;
 									}}