Usually it's stuff where the fan seems to have picked up on something implied in a story, but missed where it is clearly stated that isn't the case ... but then they go and write on fandom and make lots of assumptions from there and fill in other gaps with guesses.
It's a not-so-open secret that a lot of wikia wikis are not only vandalised but encouraged to be vandalised as to make people move off them.
Just wish there was a more centralized / good alternative to promote rather than just wrecking fandom.
I am aware of a few game communities that purposefully poison the fandom version of the wiki with inaccuracies that are non-obvious and time-consuming to verify (so they aren't just auto-reverted).
Prior to my discovery that fandom was bad and a lot of wikis were moving away, I was following so many instances of out dated info in games I was playing due to not realizing that the wiki was no longer maintained since the active contributors had moved elsewhere and updates/patches to the game had rendered the info moot.
As someone who once edited those wikis, I certainly hope they did. Who wants to work for free to enrich some private equity firm?
Same goes with just about every wiki that has a counterpart that's not on Fandom.