I was making after hours config changes on a pair of mostly-but-not-entirely redundant Cisco L3 switches which basically controlled the entire network at that location. While updating the running configs I mixed up which ssh session was which switch and accidentally gave both switches the same IP address, and before I noticed the error I copied the running config to the startup config.

Due to other limitations and the fact that these changes were to fix DNS issues (and therefore I couldn't rely on DNS to save me) I ended up keeping sshing in by IP until I got the right switch and trying to make the change before my session died due to dropped packets from the mucked up network situation I had created. That easily added a couple of hours of cleanup to the maintainence I was doing