This is more of a "Mildly Infuriating" instead of a full on nightmare.
This is also not 5e.
Anyway, we have a person who is cursorily part of our group. He left a long time ago because he's an asshole and he cheats. He also minmaxxes to a degree none of us do. When he plays it's always the same character. It's a sorcerer or spellblade type of character, he doesn't play with the group, is always invisible, and doesn't participate in any roleplay or most combat. It's really hard to explain, but he's the problem player we all know about.
Anyway, he hadn't played with us for a few years, when we all got sick of him and the group "broke up." We restarted without him. The DM, for whatever reason, feels bad for him and likes him.
After a while, the DM said he heard we were playing and wanted to try again because no one else will play with him. We said no.
Fast forward a few months, and one of our players is moving away so we need a replacement. I request to have my son join. I get that approved, but only if the problem player joined, too
It went as well as expected, he lasted a few sessions, everyone is annoyed. He cheated, never played with us, barely talked, invisible -through-the-dungeon to aggro everything. The whole time, the DM never said anything about the rolls that were twice a high as everyone the unlimited spellt slots, the massive bonuses to every skill...
Yet, I get asked for multiple copies of my characters, and my son keeps getting called out for "math that doesn't add up" for his character, and we are told that our characters always break the rules and character guidelines.
It's not a gamebreaker, but it's freaking annoying.