Playing a piano opens secret doors.
Zombies only take headshots. Crippling injuries don't effect their ability to kill you.
Shooting someone in the big toe with a really big rifle is often extremely fatal.
The best place to hide is where everone expects you to be, so they're not going to bother looking. (This one I've noticed actually works.)
No one's going to notice the brightly colored ninja running along the roof of a single story hut.
If it's evil, it will leave sentient black or red puss all over the land. (Doom III, Condemned, Prince of Persia, Dead Space, oh the list goes on.)