API Route Returns 404 After Refactoring

1. Route path changed or typo introduced in endpoint URL
2. Route handler file moved but import not updated
3. Router not mounted to app or removed from main server file
4. Route method changed (GET to POST) without updating client
5. Route registered after 404 catch-all middleware

Verify route is mounted: app.use('/api', routes). Check full path: app.get('/api/users') vs router.get('/users'). Verify route HTTP method matches client. Place routes before 404 handler. Log all registered routes: app._router.stack.forEach().