[Bozos of Bones]

If I may add:
When shot upon from a distance, hide behind a very thin and weak railing. All the bullets will infallably crash into it like it's a magnet.
The good guy is usually highly resistant to the bad guy's strongest weapon.
Lovely damsels in distress can always hit a fly a mile away with a single shot from a chaingun with no sights.
Specially trained evil snipers will always miss their good targets, or hit them in a badge that will deflect the bullet.
If a terrorist organisation decides to take the White House, there are no soldiers to offer any resistance them.
The best friend that dies early is always the main bad guy.
A hand grenade will destroy a city block if thrown by a good guy at the bad guy, and a hand grenade thrown by a bad guy will explode like a firecracker. The inverse is it may explode like a pocket nuke.
When homicidal killers are in the area, mobile phones lose signal or have an empty battery due to the killers electromagnetic nature.
Killers with melee weapons can not be hurt by any form of ranged attack.

That's all I can come up with. I'll post some more when I think of them.