The start of the final chapter when you are introduced to the final monster type. I was running low (read: only had 15 Dentist rounds) on ammo at the end of the previous chapter, and the game has locked me into the first fight sequence with exactly 7 shotgun rounds (from two barrels) and the the 15 dentist rounds I had going into the fight.
To make matters worse: if the enemy gets too close, you enter into one of the worst (if not the very worst) quick time events I've ever seen in a game. Twenty tries and I'm giving up, I just don't have the kind of ammo I need to kill it. Doesn't help that the shotgun (whatever it's called) is the one weapon I hardly ever used, so I haven't put any points into it at all.
Granted, this won't happen to a ton of people, but the conditions were just perfect to turn this into a case of incredibly bad (if not almost impossible) checkpointing. It's a shame, too, because I was really having fun with it up to that point.
Oh well, back to the rental store it goes.