Most old-school D&D-like games have a rule that limits XP gained, and thus levels gained, from a single expedition. I once removed such a rule to speed up character advancement in the early game. This was fine, in isolation.

I also had elaborate homebrewed treasure charts. Some of these could generate tens of thousands of gold coins as part of treasure, leading to tens of thousands of experience points. This was fine, in isolation.

So, once, I switched editions of a game in the middle of a campaign. This involved the game system moving from gold to silver standard, experience point rules included. This meant the next treasure of gold coins ended up 50 times more valuable than previously, which, in combination with the first change, caused the player characters who found it to shoot past several levels. Oopsie.

In the same campaign, rules for animating dead monsters were vague on which kind of special abilities undead could have. The rule was something like, for every caster level in excess of the hit dice of the re-animated monster, the resulting undead creature can have one special ability. Since there was no accompanying complete list, I allowed purchasing of spell effects to this end. Cue herds of zombie cows radiating mass invisibility and mass silence. My players were very good at milking this for all its worth.