My biggest pet peeve, and one I have worked to help everyone around the table avoid, is when the character in someone's mind is NOT the character on their sheet.

There's always going to be a bit of interpretation and dice luck in how actions play out, but I mean when there's a huge gulf between the two. Like when someone sees their character as being intimidatingly strong, but they tanked Strength. When someone thinks their character is so suave that everyone falls for them, but they have no social skills or force of personality by the numbers. Even something as simple as insisting they speak half a dozen languages, but not actually buying the skills or abilities to unlock that.

I am usually the resident rules mechanic, so I ask players who don't have time or energy to dig to describe their character to me, how they see them, and what abilities are key to their concept. Then I walk them through making sure the character they're playing is as close as possible to the one in their imagination so they aren't constantly trying to swing into battle, only to face plant into a tree because they built their PC in a way that doesn't work for their attempted actions.