Used Copilot to write a user auth migration script. It silently reset 2FA settings for ~3k users with OAuth accounts because it didn't handle NULL vs empty string edge cases in our legacy DB schema. Classic "garbage in, garbage out" situation.
Found out two weeks later when angry users couldn't log in during peak hours. Damage: 4 hours of downtime, one very grumpy security team, and a new "no AI for auth code" policy.