Invisible Warthog ammo
From Halopedia, the Halo wiki
Invisible Warthog ammo is a glitch in Halo: Combat Evolved, where driving an M12 Warthog into deep water will cause the cartridges on the chaingun belt to become invisible as long as they are under water. The gun still fires normally despite the fact that it appears to be empty. This is a simple texture glitch.
This can be done on the Campaign level The Silent Cartographer as well as the Multiplayer level Death Island on Halo: Combat Evolved for PC. However, the water found in the latter is not deep enough to cover the ammunition belt unless the Warthog is flipped.
- This is probably caused by the simple design of the rounds on the ammunition belt. They are 2D textures unlike the 3D belt.