Bullet hole glitch
From Halopedia, the Halo wiki
The Bullet Hole Glitch is a texture glitch that occurs in Halo: Combat Evolved and Halo PC. It results in bullet holes in walls that are inexplicably stained with human blood.
Halo PC
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 bumping (with a turret, for instance). Then, shoot any corner or edge with a human weapon, and the blood should appear.
