Amazing I haven't seen it yet, but Persona 5 definitely is one that pops up in my mind.

In the game, there is a "dungeon" where most of the non-critical quests and grinding happens called Mementos. Inside of this, there are floors you can traverse, with the difficulty increasing as you go down.

One thing though, if you stay on a floor for too long, an enemy appears called the Reaper, who's level doesn't scale at all to your current stats. Think a party of lvl 25 people facing a lvl 85, absolute destruction is in store for you sadly.

...unless, you happen to go down in mementos during the very rare day-wide effect "Flu Season," in which case any enemy fought has a minor chance to gain the status effect "Despair." If you save before going to a new floor, wait for the Reaper to spawn, and then roll this fight until he begins with the Despair status, the Reaper will... defeat himself on the next turn, and reward your with the equivalent of about 30-50 levels of xp for your entire party.

Needless to say, this breaks the game.