When I was training to be a copier tech I pulled out a $1000 drum without retracting some lamps, hence a nice long scratch along the surface and a fubar'ed drum. It didn't happen twice!
I tripped out an entire floors power by shorting out a power supply when testing with a volt meter that was set to measure amps not volts. I didn't admit to that one and they never suspected it was me.
I dropped a tiny piece of a solenoid that I couldn't find, (I looked for an hour). There were no replacements in local stock and it meant the copier was completely down until one was flown in from Germany. I found the piece later that evening in the turn-ups at the bottom of my trousers, (pants).
At one important customer I was doing some maintenance work on their colour printer, they said they had a mega important job to get out later that day so please make sure I'm done in an hour. Everything that could go wrong went wrong and I was there for 3 days!
etc...