Personally I've done a couple dumb ones. Guilty of forgetting to remove the barrel plug. done that a couple times. Once while doing target shooting I popped in an empty mag and tried firing...at least 5 or 6 times before I realized the gun wasn't broken, just my brain :P
My personal worst was when I'd dismantled my mechbox to do some upgrades. I'd gotten images, etc of how it was supposed to look. Opened it up, found out that Version 1 boxes are a bitch to work with and reassembled it. I hooked up my battery and squeezed off a shot (no BBs loaded), seemed fine so I didn't think anything of it. 5 months later I was at the first game for the season, took out my AEG for the first time and tried firing, only to find that while the gun was firing and you could feel the air coming out the barrel, no rounds were coming out. Tore the gun apart again to later discover I didn't put the nozzle onto the tappet plate (think that's the right part) properly. *coughs* She shoots great now though!
Another awesome one was my buddy who managed to shoot himself. We were in a forest and he was shooting at me through some trees. Well he wasn't looking while he shot and fired into a tree he was hiding behind. The BB bounced off and hit him in the leg. Never gonna let him live it down. 'specially since he's in the reserves :P
"You mean to tell me it was one guy with six guns and he was a senior freaking citizen?"