Game breaking bugs? At this point in time, there are none i can think of.
The broken server browser, that the game was launched with was a unbearable game breaker. I could only refresh two times, then fewer and fewer servers would display, until i rebooted my computer. This issue was resolved long ago, but it did kill my initial excitement for the game.
I once had to re-log into a server, because a tank crewmember joined my tank, he spawned, drove of in the tank, but i was left behind, as a man just sitting in the air xD probably happened because it was a custom map. It bothered me most, because i was shelling a house, and this one enemy teased me, told me if i got hotter or colder, still couldn't manage to blow him up.