Something I find more irritating in entertainment these days is characters who walk around with their finger on the trigger. Obviously they need to pull the trigger to fire the gun, but I'm talking about when they're chasing after someone, or walking, or sitting, and they have their finger resting on the trigger guard.
I've done a little target shooting in my time. Clay pigeons, paper targets, stuff like that. One of the things that was impressed upon me (among many safety tips) was to keep my the gun aimed at the ground and my finger out of the trigger guard until I was actually ready to shoot. That way there wouldn't be any accidental discharges. Considering most of the characters I see in comics, or movies, or whatever are professionals (cops, soldiers, vampire killers, etc.) I'd think they'd have been taught something similar. I mean, I never see people flick the safety off, so the gun will discharge if they pull the trigger.
I know the characters are typically trained professionals (and if they aren't, if we're talking some stupid punk crooks, then I'm not as bothered by their poor safety habits), but it isn't as though they couldn't trip, stumble, or be jostled unexpectedly. It really wouldn't take that much to accidentally depress the trigger, and then they shoot themselves in the foot, or one of their friends, or some other random person who happens to be in the way.