Stuff like saying Jackie's streak is natural (a lot of people take that at face value but someone on the crew as much as said the joke is that Star's wrong), Eclipsa's exact relation to Star being written incorrectly in that there are way less "greats" than there should be, a page of facts about Glossaryck where the entire theme is just "yeah I don't actually know shit about him so I just made all these up lol," probably more but I'm not looking at the book right now.
but I didn't mean that post to be "Star's always wrong" or anything; more that it's a lesser canon source than the show itself and Star as a character/source of information isn't always entirely accurate. so when it comes to things in the book that aren't referencing elements of the show we know for sure because the show has illustrated them onscreen (ex. cheek marks developing through magic, as in Marco or Festivia), at most we can just say "Star maybe implies Mewberty applies to all girls, but there hasn't been a definitive answer given in the show yet." some people even debate whether she's just referring to her own family too, so it's still pretty vague
if it was written by a more reliable in-universe source there wouldn't be a debate in the first place, ex. no one questions the stuff like the names of Star's classmates or the "wanted" posters in the book, because it's not just Star writing what she thinks she knows in those instances