Glitched in Infection Lobby: There is a bug if you join infection a bit before the next round starts at the lobby you stay in the lobby and can't play the round. Ladder Block + Pushing in 1.9: When people play in 1.9 they can get pushed around by 1.8 people, making it unfair because 1.8 people can block ladders and 1.9 people can't get through. Also on parkours on the maps 1.8 people push 1.9 people off the parkour when they jump on the same block as them. I'm not sure how this will be fixed though. These were some more problems I found in infection, the other problems were mentioned in another thread.
