Edge Case Analysis: Subscription Checkout

Current AC Coverage: 33% (happy path only, missing critical payment, error, and security edge cases)

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Missing Edge Cases Identified (14 cases)

Category: Empty/Null States

Edge Case 1: What if no plans are available to display? MEDIUM PRIORITY
Current Status: Not defined
Suggested AC:

• “If no plans exist, show message: ‘No subscription plans available at this time’”
• “Display ‘Contact Support’ button with link”
• “Log incident for investigation (should not occur in production)”

Why Important: Graceful handling prevents broken UI and user confusion

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Edge Case 2: What if user has incomplete payment method from previous attempt? MEDIUM PRIORITY
Current Status: Not defined
Suggested AC:

• “If incomplete payment data exists, allow user to complete or start fresh”
• “Show message: ‘Continue with previous payment method or enter new one’”
• “Clear incomplete data after 24 hours”

Why Important: Reduces friction for users returning to complete checkout

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Category: Boundary Conditions

Edge Case 3: What if plan has $0 price (free trial/free tier)? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Free plans ($0) skip payment entry screen”
• “Show confirmation: ‘Your free plan is active’”
• “Still collect email for account creation”
• “Allow upgrade to paid plan later”

Why Important: Common business model, prevents confusing payment flow for free plans

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Edge Case 4: What if plan price is very large (e.g., enterprise $10,000+)? MEDIUM PRIORITY
Current Status: Not defined
Suggested AC:

• “Display prices clearly with proper comma/period formatting for locale”
• “Show price breakdown if applicable (per user, per month)”
• “No arbitrary maximum price limit”

Why Important: Professional handling for enterprise pricing

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Category: Error Scenarios

Edge Case 5: What if payment fails (declined card)? CRITICAL PRIORITY
Current Status: Not defined
Suggested AC:

• “Show clear error: ‘Payment was declined. Please check your card details or try a different card.’”
• “Allow user to retry with same or different card”
• “Do not create subscription if payment fails”
• “Log decline reason (insufficient funds, invalid card, etc.) for support”

Why Important: Most common payment failure, needs clear recovery path

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Edge Case 6: What if payment gateway is down/unavailable? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Show error: ‘Payment system temporarily unavailable. Please try again in a few minutes.’”
• “Provide retry button”
• “Send alert to ops team”
• “Do not charge user or create subscription”

Why Important: Prevents bad UX and billing issues during outages

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Edge Case 7: What if card number is invalid? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Validate card number format client-side (Luhn algorithm)”
• “Show error immediately: ‘Invalid card number’”
• “Highlight card number field in red”
• “Disable payment button until valid”

Why Important: Prevents unnecessary API calls, improves UX

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Edge Case 8: What if card is expired? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Validate expiration date client-side (must be future month/year)”
• “Show error: ‘Card has expired. Please use a different card.’”
• “Highlight expiration field”

Why Important: Common issue, caught early saves API calls

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Edge Case 9: What if CVV is missing or invalid? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “CVV field required (3-4 digits depending on card type)”
• “Show error: ‘Please enter valid CVV (3-4 digits on back of card)’”
• “Mask CVV input for security”

Why Important: Security requirement and common user error

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Edge Case 10: What if user already has active subscription? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Check for existing subscription before allowing selection”
• “If active subscription exists, show: ‘You already have [Plan Name]. Would you like to upgrade/downgrade?’”
• “Prevent duplicate subscriptions”
• “Offer plan change flow instead”

Why Important: Prevents duplicate billing and user confusion

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Category: Concurrent/Conflict Scenarios

Edge Case 11: What if plan pricing changes while user is on checkout page? MEDIUM PRIORITY
Current Status: Not defined
Suggested AC:

• “When user clicks confirm, validate current plan price matches”
• “If price changed, show: ‘Plan pricing has been updated. New price: $X/month. Continue?’”
• “Require user confirmation before processing changed price”

Why Important: Transparency and legal protection for price changes

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Edge Case 12: What if user submits payment twice (double-click)? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Disable payment button immediately on click”
• “Show loading spinner: ‘Processing payment…’”
• “Implement idempotency key to prevent duplicate charges”
• “If duplicate detected, show existing confirmation”

Why Important: Prevents accidental double billing

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Category: Input Validation

Edge Case 13: What if user enters SQL injection or XSS attempts in payment fields? CRITICAL PRIORITY
Current Status: Not defined
Suggested AC:

• “Sanitize all inputs server-side”
• “Use parameterized queries for database operations”
• “Payment provider SDK handles card data securely (PCI compliance)”
• “Log suspicious input patterns”

Why Important: Critical security requirement, PCI compliance

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Category: Network/Performance

Edge Case 14: What if payment processing takes very long (30+ seconds)? HIGH PRIORITY
Current Status: Not defined
Suggested AC:

• “Show progress indicator: ‘Processing payment… This may take a moment.’”
• “Timeout after 60 seconds with error: ‘Payment is taking longer than expected. Please check your account or contact support.’”
• “Do not show success until confirmed”
• “Verify subscription status server-side before showing confirmation”

Why Important: Prevents confusion during slow payment processing

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Priority Summary

CRITICAL/HIGH (11 cases) - Must address:

• Payment declined handling
• Payment gateway unavailable
• Invalid card number validation
• Expired card validation
• Invalid CVV validation
• Existing subscription check
• Double-click/duplicate submission prevention
• SQL injection/XSS prevention
• Free plan ($0) handling
• Payment timeout handling

MEDIUM (3 cases) - Should address:

• No plans available state
• Incomplete payment method from previous attempt
• Plan pricing changes during checkout
• Very large enterprise pricing

LOW (0 cases) - Nice to have:

• (None identified)

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Coverage Improvement

Before: 3 acceptance criteria (happy path only)
After: 17 acceptance criteria (3 original + 14 edge cases)

Estimated Bugs Prevented: 10-14 (including security vulnerabilities and payment failures)

Recommended Next Steps:

1. URGENT: Review security and PCI compliance edge cases with engineering and security teams
2. Implement payment validation and error handling (critical/high priority cases)
3. Test with test credit cards for various decline scenarios
4. Ensure idempotency for payment submissions
5. Add monitoring for payment gateway failures