Gwyn in Dark Souls.

The problem was my biggest success. I managed to score a zweihander, pretty early on in the game. I upgraded the hell out of it and was able to clear most bosses with some grinding and a lot of determination. I did this with a failing PS3 controller that generated spurious inputs, resulting in randomly switching to my alt config (unarmed) if I squeezed the controller too hard. I even learned a new level of personal zen and patience after realizing that getting angry would make me perform worse. I got gud.

Then I arrive at the end boss and get absolutely wrecked.

Why? I never had to parry. Not. A. Single. Time. Strike, juke, medium-roll, retreat, strike again. Don't be a target, and keep moving. Meanwhile, every guide to beat Gwyn says you have to parry his attacks. I basically had little choice but to retrain how to play my character modifying an entire game's worth of muscle-memory in the process.

My save file has been in this condition for years at this point.