Not mechanical, but:
Doing monthly maintenance at a remote multimillion dollar field installation, there was a UPS system that wouldn't turn on and maintain the acquisition of data during power failure testing. Attempts to reboot it remotely had failed all month and it had a bunch of warnings and errors. Keep in mind this was supporting a rack of servers and storage NAS and was remotely controlled from our office in Europe.
After trying all kinds of things on site, I wasn't having any success. In an attempt to load it and see what it would actually do, I plugged in the shop vac that was there to keep things clean. Surge current during motor startup wasn't really on my mind at the time, but there was a puff of smoke, smell of burnt semiconductors and the UPS audible warnings squealed. I grabbed the off switch on the shop vac, figuring I blew the whole setup.
After things settled down, I started looking through the system only to find everything working properly and, after rebooting on its own, the UPS came up without errors. I ran it through an proper power outage test, which it passed with flying colours. It worked perfectly for the remaining 18 months that project ran. No idea why it didn't fry.