Support tickets Friday afternoon: "Can't log in with Google." Error: redirect_uri_mismatch.

Mistake #1

We moved from HTTP to HTTPS in production. Updated the redirect URI in code but forgot to update the Google Cloud Console OAuth settings.

Mistake #2

Our load balancer terminates SSL. The app saw incoming requests as HTTP and built HTTP redirect URIs. Google rejected them because we registered HTTPS only.

The Fix

const redirectUri = process.env.NODE_ENV === 'production'
  ? 'https://app.example.com/auth/callback'
  : `${req.protocol}://${req.get('host')}/auth/callback`;

Better: move all redirect URIs to environment variables. One config per environment, no hardcoding.

Lesson

OAuth errors tell you what's wrong, not why. Always update both code and provider settings. Infrastructure changes what your app sees—never assume request protocol matches deployment.