Here's a couple, one where I lucked out and one where I didn't.
I was restoring a TR4 awhile back and rebuilding the clutch slave cylinder. The piston wasn't coming out so I put some a little compressed air to it. Still nothing - so I gave it a good blast and sure enough POP out flies the piston. It headed eastward through my shop about 30 feet, right where I had all of the freshly painted body panels stored. The were leaning against a glass fronted cabinet. Well, it's amazing but this little projectile landed on the cabinet ledge, right behind the body panels and right in front of the glass panes of the cabinet.
One instance where I was not so lucky was when I had a 63 Buick Riviera parked between my 2 post lift. The car had just been restored, a beautiful black on red example, it was already sold and waiting to be picked up the next day by the transporter. I decided to move it from the lift and get it near the door for tomorrow's pickup. I've moved countless cars in and out from between these 2 posts but for some reason I misjudged this one. I'm moving slow and all of sudden I hear a sickening SCRUNCCCHHH sound. Yep,I scraped a good 16 inches of paint off the passenger door and created a nice dent for good measure. Turned off the car and got out to see what was in store for me. It was one of those moments where you really can't believe you were THAT stupid, and of course you only have yourself to blame. I picked up the phone and dialed my customer on the opposite coast to tell him what happened, thankfully he was the good natured type and said he'd have his paint guy take care of it. 3 weeks later I mailed him a check for $700, that was an expensive mistake.