JWT Token Validation Failing Unexpectedly

1. Secret key different between generation and validation (different environment, code change)
2. Token expiration time too short or checking incorrectly
3. Signature algorithm mismatch (HS256 vs RS256)
4. Token claims (sub, iss, aud) don't match expectations
5. Clock skew: server time different from client, causing expiration validation to fail

Verify secret key is consistent. Use JWT.io to decode token and inspect claims. Check expiration claim: 'exp' is timestamp. Allow clock skew: validation can tolerate small time differences. Ensure algorithm matches: if generated with HS256, validate with HS256. Log validation failures with details: which claim failed, expected vs actual.