Epic Levels

When Dungeons & Dragons 3rd Edition came out, the rules only went to 20. Eventually people wanted more and Wizards printed the Epic Level Handbook to allow higher levels. It was retardedly broken, and hated martials even more than normal 3E. While epic level Wizards got feats that could quicken every spell for free, cast more spells than 1+1 quickened, or use "epic spellcasting" (more on that mess in a bit), a Fighter got feats that gave him +4 damage if the target was within 30 feat. Fighters didn't even get higher attack bonus than anyone else when advancing past 20!

Perhaps the biggest mess is epic spells. "Despite their power, epic spells still follow the basic rules for casting spells, except as specifically noted otherwise.", with the "noted otherwise" being pretty much everything except how Dispel Magic works on them. Unlike normal spells, epic spells are all created by the player (though there's a handful of really terrible examples). Not the player character, the player. Epic spell are essentially a big custom spell system that you need to meet a skill DC (very easy to munchkin to a far higher level than intended) to make and use. Unlike most "custom ___" systems, these aren't suggestions or guidelines (except in the way all rules are), a caster can do anything they can get a high enough skill check for.

The one thing that people actually use out of these rules are the skill uses, in large part because they aren't actually tied to epic levels and can be made by normal characters (albeit with some focus required). With very high ability in a skill a character can create truly impressive effects. The most infamous uses are the Diplomancer, able to make any enemy a fanatic ally in seconds, and Escape Artist allowing you to go through spaces smaller than your head, potentially even other people. There's even uses for skills that should have been in the core book like hiding someone or avoiding suspicion when asking for information.

4E avoided segregating epic rules and instead used Epic Destinies to control high level progression.

Pathfinder went another, though equally broken (both in the "overly powerful" and "this shit doesn't work" sense) idea with Mythic Adventures. Rather than increase level directly, a character got additional "mythic tiers" that gave him new abilities. Like Epic Level Handbook these abilities varied between stupidly OP and weak even for normal options. Also like ELH, most of the stupidly OP stuff went to casters and the weak stuff went to martials: Casters can become a demigod capable of granting spells, martials can move and attack at the same time.

Paizo pretty much dropped it when they realized how shit it was, but it did survive for one use: While the system is horrible to give your PCs, it's really good for making boss monsters. While giving players extra actions, immunities and powerful new abilities is a good way to destroy your game, boss monsters enjoy being able to act multiple times against a group of multiple PCs, not getting save or died, and having some flashy abilities random encounters won't have. Even Paizo acknowledged this, as within a year the only time they ever referenced the book was some boss monsters.