Code Veronica as a whole, pretty much. It's the hardest classic in my opinion, and also the only RE where I didn't have an easy mode to pick for my first playthrough - bad combination! I already felt the despair when the game started throwing those yellow monsters at me, pitting fast, long-ranged arm attacks against slow tank controls. And Claire couldn't even evade or turn around quickly like Jill in RE3.
Another terrible experience was the train crash on my first playthrough of Zero a few weeks ago. I knew it was going to happen eventually, but not when, so I kept the most important items in my inventory at all times, even if it meant a lot of backtracking, because I didn't want to lose them. When the train did derail, I nearly freaked out because I had dropped the shotgun for some reason just before it happened, and I was about to rage quit and restart from an earlier savegame later. Then, thank the non-existent God, I realised all the items I had picked up on the train had fallen outside with me, and I continued playing. Nowadays I'm even amused at how shocked I was back then, but I remember how I already started phrasing the worst review ever in my head because I thought the game had stolen my shotgun. :angry:
What do we learn from this? Never steal Hel's shotgun!