A banking app silently logs users out after 15 minutes of inactivity:
❌ BUGGY PATTERN
// Server-side timeout, no warning
const SESSION_TIMEOUT = 15 * 60; // 15 min
// When time expires: redirect to login
// User loses unsaved work!
Result: User filling out a form (takes 20 minutes). Session expires silently. Hits Submit and gets "Login expired" error. Form data lost.
Issue: No warning. No extension option. Accessible users (slow typists, cognitive disabilities) are most affected. Users lose work without notice.