What great game did you abandon because of a single badly designed moment?

Recently I really liked playing Signalis. It's like Resident Evil 1 in space. I was really enjoying it until the first boss fight. The first boss fight is against a mechanical monster that seemed to take no damage whatsoever, while spawning other lesser creeps. In this game the ammo is very sparse, so I ended up using all my ammo and still failing. I watched a let's play of it and it turns out, you just have to wait for the boss to go into a vulnerable state. I felt like the game did not communicate this well enough and I have not gone back since.

Another great game, Legend of Grimrock, an old school dungeon crawler with tough fights and even tougher puzzles, had these enemies that took 0 damage from my hits. I thought that maybe you need a special weapon or something, but it turns out that you just had to hit them over and over again until at some point they magically start taking damage. Again, not communicated well enough. In this case I finished the game but my opinion on it was tarnished.