Infection Spawn bug on Ice Research map.

[Johney]

So basically when an infected spawns in the upper original spawn area, due to them having less gravity as infected, the spawn area forces them to stay up instead of letting them float down. This results in the infected being stuck trying to go down until someone comes and kills them, thus giving the person that killed them the possiblity of getting like 7 kills just on the one wave trying to get down from that area.