Bullet hole glitch
Fire a bullet from a human weapon nearly parallel to any surface that will leave a bullet hole (i.e. a wall or hard ground, not dirt or ice). At the point where the bullet hits, there will be a mysterious splotch of human blood.
Halo: Combat Evolved
Exit a level's boundaries by means of wall hacking. That is, make the game move Master Chief past a wall of some kind such as by performing an exiting movement (with a Shade, for instance). Then, shoot any corner or edge of an object's surface with a human weapon, and the blood should appear in the bullet holes.