Hrm, just thought of another good one too...
A few months back I just happened to be watching LEM, I guess because I am weird that way and I saw a ton of failed logins for one of our engineers Active Directory user accounts. It was clear that it was some sort of service account that started failing due to him changing his password. Now, in sysadmin 101 we all learn that you use service accounts for that stuff, not your own personal account so I had to go have a talk with him about misuse of his account and then also let him know that it was failing. He was a bit ashamed of himself for getting caught on such a bush-league mistake.