WITH A FIREARM!!!!!!
Stop giggling and answer the question.
I'll go first.
I've had 2 in 20 + years of carrying a handgun as a professional in one capacity or the other.
1st one - a Bauer .25 ACP, my fault, 1 round into a box, then concrete floor.  I carry the case and the flattened projectile to remind me of what happens when you get stupid and careless.
2nd one - a Colt 1903 Pocket Model .380 ACP.
Picked it up off my desk to begin repair, checked chamber (opps, I skipped a step, didn't I?), released slide, pointed it at my stool beside work bench, pulled trigger BOOM! Hole in stool.  I carry that case also, never recovered the projectile.
As you can probably imagine, I don't like old striker fired autos much anymore.

BTDT - check my story on Gunnery Net.
Here's the link Shot my own leg

So message of the day: Kids, don't try this at home.

C-2-6

No NDs for me fortunately. <knock wood>