Yeah, in Act 4 during the morlock tunnels mission the game crashes and freezes, one of my friend has the exact same error and I have found people on the net with the same error. Apparently its a rare error and its been fixed on the PC version but the console versions shouldn't have game ending bugs.
Activision have tried to convince me that my save game is corrupt but thats B.S. I can still play all the other missions available and if the save game was corrupt then it wouldn't load in the first place.