Many many years ago, broke a cardinal rule of gun handling in not making sure one was unloaded.
I'd been out at the range with friends, and so that evening I had several that needed cleaning.
Was also watching TV and got to cleaning one of the .22 pistols.
To disassemble that particular model the trigger has to be pulled so the striker spring is relaxed.
So, I pointed it at the floor and was taking up the slack on the trigger when a little voice in my head screamed "CHECK IT!!!" so I racked the slide and a shiny live round popped out onto the carpet.

Damned if I didn't just about plug a round into the apartment below me. I don't know that a .22 would have made it through the carpet, padding, subfloor, insulation, drywall and into the room below, but I sure would've had a lot of explaining to do.

That scared the crap out of me, and from then on, when ever I clean guns I always check all of them at the start of the session, then each one as I get to it.
And no live ammo or loaded clips are allowed in the same room while I'm doing this.
If I leave the room and come back, I check them again. Even if I'm the only one home; it's just a good habit to be in.
Sometimes you
can learn before something really bad happens.
-K