I ain't got nuthin' on the stories told so far but one that comes to mind that's kind of embarrassing takes place a lifetime ago.
I'm fresh out of high school and start a company with my parents remanufacturing toner cartridges and repairing laser printers.
On one of my early repairs I'm at this fairly large corporation (I don't recall the name of it now but they owned their own building that was 3-4 stories tall). The floor I was working on was a call center. There had to have been at least 50 cubicles set up and all of them chattering away. I had to replace the fuser assembly in an HP LaserJet II. Easy. 4 screws.
I open it up and start unscrewing the fuser assembly and all of a sudden *ZAP*!!! Huge, bright, crackly sparks, I get the shock from hell and the power goes out for the entire floor. Complete darkness.
Then I realized I was trying to replace that part while the printer was still turned on. D'oh!
Thankfully everyone had a good laugh over it and they continued to be a good client. When we got the power up and running again I saw the screwdriver I was using sticking straight out of the printer. It had been fused to the screw I was unscrewing.