Forbidden banished exiles. Especially in MMORPGs, I've found that the most common overall in-setting guild theme is "we don't belong in the society that this setting established." It's not just in MMORPGs, of course. It's often a crutch (or training wheels, if I'm being generous) for some roleplayers to participate by not participating. I'm not saying that a character (or a player for that matter) has to bend the knee to Lord So-And-So, but it's kind of hard to find plot excuses for the the forbidden banished exile to even be in the same picture, let alone get the quest to do the thing.
Plot armor dependents. Yes, it's quite likely that if I'm running the game and telling the story that I won't have the guards kill a player character on sight even if they do something exceptionally stupid just to see what happens, like stabbing a random citizen. Those kind of players don't tend to last long if their only contribution is "try to knock down the props just to see what happens on set."
People that directly lift an established character in well known fiction, often just changing the spelling, and often not even trying to resemble or act like that character. In an older MUD, I recall a "Frrodoh Bhaginz" that was a Half-Ogre hunter. Again, it didn't ruin the game, but it was annoying. I admit it can sometimes be a red flag warning, and a helpful one, if the lifted name is from chud fiction, like if they have "Rahl" in their name. It can show me what to expect, either in a tabletop group or in a MMORPG guild.
Spending 10 minutes looking up a rule or item or whatever that nobody expected to need or bothered to prepare. It's very flow breaking to be mid battle and someone goes, "oh I have the magic amulet of such and such from 8 sessions ago, what did that do again? I think it's useful here"
As a DM I always establish that I will make something up that makes sense at the time and for next session we will use the real rule if it's different. I feel this makes players be more conscious of what they can do as sometimes my interpretation of the item is incorrect and less powerful which nobody enjoys.
Some players hate this, but I maintain a similar policy for purposes of game flow.
If someone is opposed to such a thing, I think it's reasonable to expect them to read everything about their character that they want to use. Obviously if it's important or a complicated situation people can look stuff up but I'm not interested in whether I got a +/-2 modifier wrong in the moment.
Luckily I only play with friends so it's never a big deal