Last week I smoked a customer's cylinder prox because I wired it to the terminal block wrong. It was one of those little Bimba proxes. Thing was half melted.
Once I touched a Devicenet cable and the entire machine went down. Although that's pretty much to be expected with DeviceNet. I learned to never touch anything on a Devicenet Network while the machine is running.
All the times I misdiagnosed something and had my employer change a part that didn't need to be changed. Now I have a policy where if I can't determine which part of a system is broken, I replace the cheapest parts first (cables, connectors, etc.).
What I've seen other people do:
This guy who worked at my last job had been a fourth year electrician apprentice. His career ended shortly after not being able to trace a circuit to shut it off and opting to cut the entire conduit...live...with a pair of large cutters instead. I normally wouldn't believe such stories except that he was the one who actually told it.
Another time, this same guy couldn't figure out why a drive wouldn't run. He's very much a "do SOMETHING, ANYTHING, until it works" kind of guy. His method of troubleshooting is "change parts until the problem goes away." By the time I got to the machine, I opened the door to see a contactor, hanging in mid air by the wires, which were attached to the line terminals on the drive with the power coming in, and the motor leads on the other side of the contactor. He used a selector switch that he had likewise thrown into the panel to manually turn the motor on and off. The problem? The analog output card on the controller was bad.
But by far the most expensive mistake I've ever seen was at a systems integrator I was doing CAD work for. We had a few systems with a DeviceNet (ugh) network. The system was set up with a main PLC rack, and a remote I/O rack on each zone all networked together. Each piece of equipment had a good dozen or more remote racks on them. The main processor for each system was a Control Logix processor. The engineer ended up ordering a Processor for every remote rack, on multiple machines. Now, one of these processors will run you a good $5,000 or more, at least, that was true back then. Now imagine ordering 50 more than what you need. In case you don't have your calculators out, this is a $250,000 mistake. The engineer was fired (being 45 minutes late to work every day on top of that doesn't help things). The A-B rep had to order extra stock from the factory to fill the order, so even though the processors could be returned, that's one hell of a restocking fee.