I have other examples, but one specific level on the hardest difficulty that comes to mind almost immediately is "Heart of the Reich" in Call of Duty: World at War. You could probably get through the rest of the levels on Veteran without THAT much exertion, but the part of "Heart of the Reich" where you storm the courtyard outside of the Reichstag, destroy four FlaK 88s, then make your way to the building is unnecessarily punishing. Let's just forget the fact that the bullets and grenades become realistically hyper-lethal and that enemies become possibly infinite if you don't advance to your objective, but the grenades also have an increased "kill hitbox" to the point where even being a fair distance from a grenade will gut you like a fish. The enemies must've also reached this conclusion, because most of the time they abandon their guns to devote their ENTIRE attention to spamming you with an off-camera box of grenades (you will usually be surrounded upward of 3-4 grenades on short fuses at any point you wish to take cover or move from cover). I actually spent most of the level just throwing back all the grenades around me (running away when they exceeded 2) until my allies finally decided to top collectively take their thumbs out of their asses and advance half an hour in (not to mention multiple replays when I was trapped behind cover by the notorious 6-grenade-combo). It all actually makes me wonder what I looked like double backing between cover to cover from grenades from the Nazi side of the fence...