Grinding out hell levels in Everquest.
Leveling was already a pain in the ass as you got higher, taking hours to grind each level since mobs gave shit experience and questing wasn't a viable way to level. For the first few expansions there was a bug that made it take multiple times longer to finish certain levels (39, 44, 49, 54, 59) than the previous level before going back to normal, so you would sit at the same camp and grind for an entire day or more, killing the same thing over and over.
Eventually it was fixed and the levels were smoothed out in terms of how much experience it took for each additional level, but god damn that was the worst. Especially since the bug made it where you lost more experience than normal if you died during the level after a hell level.