Bullet hole glitch
From Halopedia, the Halo wiki
In Halo: Combat Evolved and Halo PC, a glitch exists in which human bullet holes will result in the the appearance of human blood.
In Halo PC
Basically, shoot a BULLET (not a rocket, needle or plasma bolt) nearly parallel to any surface that will leave a bullet hole (i.e. a wall, hard ground, not dirt or ice). At the point where the bullet hits, there will be a mysterious splotch of human blood.
In Halo: Combat Evolved
First, get outside of any levels playable area(i.e. outside the map) by means of bumping(with a turret for instance. Second, shoot any corner or edge with a human weapon, and the blood should appear.
