Scenario 24: Integrating with a third-party OAuth2 provider, you keep getting 'invalid redirect_uri' errors even though the URL looks correct.

redirect_uri must match EXACTLY, character-for-character, what's pre-registered with the Authorization Server — a trailing slash, http vs https, or a different port is enough to fail the match. Carefully compare the registered URI and the one your application actually sends (log it) to spot the discrepancy, which is very often a subtle formatting difference.