Shooting yourself in the head can summon demons. No really, I tried it. It works. (Persona 3)
You can die instantly from getting shot in the leg or arm. This also means you can get shot a couple times in the chest and be fine. And if this is a casual shooter, you can get shot between the eyes, wait a few seconds, and you'll be healthy yet again. (Most FPSs)
That the MC from Doom, Doom Guy, is probably the strongest video game character ever
What? I have 75 rounds left? That must mean I have at the most, 75 magazines left. *Fires a bullet* *Reloads a new mag* *Throw away old mag only missing one bullet* (All FPSs)
Belts and zippers and skimpy armor are apparently good defense against monsters. (JRPGs and Monster Hunter)
EDIT, forgot one: Tapping spikes, even the most gentle tap, will blow up a robot. (Megaman)