Killed a cheap plastic Panasonic by dropping it and cracking the case. Wore out the switch on another one (man, they bend and loose contact easily)
Instaflashed bulbs experimenting with overdriving.
Then there's playing 2 person tug of war with an old Garrity "Tough Light" at camp and snapping the rubber lanyard in half. The light was still working when I lost it about five years ago.
I would say the only thing I've actually worn out was an old MiniM@g. I got it from my uncle when I was young and used it a lot until one day, the bulb burned out and I couldn't find the spare one in the tailcap since someone already used it. I lost that light somewhere in the house. Still, I'll someday get another one.
Broke some contacts in an Inova Microlight. The light still works but not the constant on switch. It needs new batteries though (I EDC it along with another cheaper keychain light that is amber in color)
Rubbed off the lettering on a Coleman Graphite and cracked the rubber button over the switch. Still works though. Oh, and there's putting my soldering iron down too close to it and melting a small dent in the rubber covered head.
I still have this old light my aunt had that I called the "Air Raid Siren" light as it had a mechanical siren on the back. Had some batteries leak in the light which corroded the contacts. It still works though. It's gone through one bulb change; it uses screwbased bulbs. Then there's breaking one of the latches off on the battery door; I tried to krazy glue the latch back on but the door fits loosely. Right now, it's held together with tape.
I've worn the lables off rechargeable batteries from constantly topping them off.