Infinity Ward knows about the unlimited ammo bug in Modern Warfare 2 and is working hard on a solution.
"Yes, it'll be eliminated soon enough," the developer's community liaison Robert Bowling said on his Twitter.
The bug, spotted by increasing numbers of gamers on forums (thanks, Kotaku), breaks ammo counters and in some cases removes them entirely - meaning the bullets never run out.
Apparently this originates from modded consoles but can spread to non-tampered machines while playing online. The bug manifests differently for obedient gamers, however, who'll have to reboot their machine and clear the cache to rid themselves of the glitch.