Tales from the field:

One developer walked into the client site, laptop bag under his arm. The moment he walked in, some hardware went down. They all turned to look at our developer shouting "WHAT DID YOU DO!!" The common rule at all clients seems to be to blame everything on the external developer, even when they're still standing in the door opening.

One techie at a different client had to install some new empty rack in the server room. He didn't found any outlet for his drill, and he didn't want to run an extension cord, so plugged the drill one in the only outlet he found. It was the outlet marked by a different color because it was linked to the UPS, and that UPS went down hard when the drilling started, taking all the servers with it. That recovery took some time.