A week out of college, at my first real job. I was assigned to shadow a more senior engineer as she debugged some code on the Navy trainer we were developing. After she spent a couple of hours chasing a bug and writing in new code which should fix it, I turned my chair and my knee hit the circuit breaker panel on the instructor station, which removed power from all of the classroom computers and lost her fix. I could tell she was furious, but she kept saying (through clenched teeth), "It's okay, it happens to everybody," as she went off to take a long break. Nothing like making a good first impression. 
