I have a few.
I ran an Aladdin's Castle many years back. In the back of the store, there was an "upper level" that was only three stairs up. I was shuffling some games around one morning, I had an upright (I can't for the life of me remember what it was now) strapped to our furniture dolly. My assistant was making change for somebody and I'm an impatient guy, so I was pulling the game up those three stairs solo. (Which, honestly, should not have been an issue at all). Well, the rubber trim on the stair broke loose and caused me to stumble, pulling the game down on top of me. I was pinned, but not hurt at all. My assistant rushes over and I'm laying under it, laughing.
I was servicing a Soul Calibur 2's monitor. We had just gotten the chassis back from the shop. I don't recall if it was a dedicated cabinet, but I'm wanting to say it was a conversion job. The only access in the back of the cab was near the bottom of the cabinet, and only about 2 foot tall. I had to press my shoulder to the back of the cabinet, then reach in and up. Make your arm like a cobra as it's reared up. We had the d*&# thing in, working, just needed fine tuning. Reached back in and it bit me. HARD. The game threw me about 3 feet backwards and ... blew the chassis. Luckily I had my other hand in my rear pocket. Remember, safety first when you're doing something stupid!
My last one happened just recently, as a private collector. My wife and I were loading a Revolution X into a trailer with no ramp. I have NO idea why this particular cabinet has this design, but the entire marquee box is on a hinge, so that it can swing forward. We didn't realize this "feature" until ... loading the cabinet, I'm on the ground doing the lifting, leaned the cabinet 45 degrees toward me and lifted. The marquee box swings forward and cracks me on the top of the head. Somehow, I didn't drop the game. There was blood oozing through my hair. The particle board was completely broken in half and the plexi broken.
Bonus: The arcade's Basketball game had a loose 5v somewhere in the coin door. The game frequently had ticket jams. The fastest way to clear the jam was actually through the coin door (I don't remember the specifics why it was now). So, clearing the jam while getting a nice 5v shock. We never bothered to fix that one. The established crew laughed about it, and it was right of passage for the newbies.
Awesome stories!