For example, I'm looking in EverQuest -> Hunter -> Hunter of Jaggedpine Forest (as an example), you log to character select and log in a different character, open the achievement window and go to that achievement, and the wrong checkboxes are checked, being shown for the previous character. Makes it really confusing about what mobs you need to kill for Hunter on which character. Work-around, as usual with the achievement checkmark bug, is probably to view a completely unrelated achievement category, then go back to the one you need to view. Bugs with the achievement checkmarks are a longstanding issue, but I haven't noticed if it's been pointed out that switching characters causes a real problem.
If you are saying the checkmarks being shown are actually the ones for another character on your account, I have never seen this happen. I have seen, however, Quest bleed over where steps are shown from other characters (and even unrelated quests), but they seem to have been fixed. I have also seen where check marks were not updated properly, but I have been able to force them to update in one of three ways... 1) Zone. 2) Check and then uncheck the completed box. 3) Drag the Achievement window to a different size.
I think I remember seeing this reported a long time ago, not sure, but it sounds kind of familiar. It's an issue with the cached character data because achievements are loaded after a character logs in, but it's likely displaying whatever was last in memory before the screen is refreshed.
