Hmm...I've done several bad things to some rather good guns...here goes.
MANY years ago, I had a Colt Commander, series 80. Parkerizd gun, carried it IWB during a very hot summer. Came home and found the entire slide had rusted from my sweat. Got out the dremel, polishing wheel and compund and figured I'd de-rust the gun. Turns out I de-finished the gun and wound up with THE WORST looking 2-tone setup ever imaginable.
Next, a friend of mine gave me 100rnds of reload .45acp ammo and asked me to try them out and tell him what I though. About 40rnds into things, my P220 went BOOM. Cracked frame, extractor gone, and badly damaged slide. Figured I may have had a squib and didn't realize it, I continued firing the rounds thru a custom Springfield 1911. Hit the 90th round and the 1911 lost its ejector, extractor and locked shut. I didn't know much about firearms at this time in life, however i examined the brass and many of them were blown to hell. Asked my friend what he did, and he said he was trying to make factory ammo "better" by "modifying them". Basically he had no clue, I took his word on things and it cost me about $2,000 to learn the lesson about backyard reloads.
Last (and most recent) I have a two-tone P220, nickel plated slide, I carried it as an LE duty weapon until recently. About 2 years ago I was assisting a local boro with a house to house search and had my weapon out. I slipped on ice and braced myself against a brick wall. Braced myself with my gun\gun hand. After work I was putting my P220 in my off-duty rig and noticed I had ground the nickel plating off the side of the gun against the brick wall. Needless to say I was peeved bad, cost $75 to get it brought back to speed. Those are the best ones I can recall...