Game: Halo reach
Issue: Long Night of Solace scoring ends (all kills generate 0 points) around 9/10 into the mission. Happened twice, first time didn't notice when it started but around the part where the ally frigate blows up and second time at start of the final standoff with Jorge. I believe it started with the first red elite (legendary difficulty) who I began shooting and killing extremely early, like right out of the tunnel with a needle rifle. I've read that the same issue occurred on the 360 version when people interacted with enemies too early or something, hitting them as they spawned for example.