I recall one HP writer who had made a terrible misunderstanding about the meaning of "starkers"
I read an Angel fic entitled 'driving Angel starkers' - in which Spike is attempting to drive Angel nuts. Of course - Spike is a British character so ... that isn't how he would use 'starkers'. I told them (along with nice things I said about the fic) that their title could be interpreted as 'driving Angel naked' - they were very amused.
I once read a HP muggle au fic where, bless them, they had heard that 'public school' meant 'private school' and so had come away with the idea that therefore state schools were called 'private schools'. They even left an author's note about how this was a correct Britishism. I left that one alone ...
I find graduation ceremonies at Hogwarts pretty irritating.
But I've been pulled up on stuff that isn't a small mistake (or a non-mistake they think is a mistake) I've recently had someone leave a review saying 'I didn't like it when Angel allowed all those people to get massacred.'
I went back to check - I figured maybe I'd written it in a way that was unclear but ... I dunno, I think the way he tells his team to 'kill them all' (the demons) and then later all the demons are dead is pretty clear. Literally no humans die once Angel turns up.
I also once had a person insistent that I must be writing an AU because The Initiative in Buffy already knew about vampires before Buffy joined them. ..They're a vampire hunting unit!
I pointed this out - they insisted The Initiative did not know they were hunting.
I pointed out that in the show they refer to vampires by name as early as episode 7 (and my story was set past that) he (of course he was a he) insisted that they didn't really know they were vampires they were using that word as a joke.
I pointed out that people in The Initiative have heard of the slayer before they meet Buffy - they just don't believe in her. He ignored that.
I pointed out that in an Angel flashback we find out that the 'Demon Research Initiative' has been around since the war and they knew Angel was a vampire back then. He insisted they weren't the same people. As a teacher, he said, he taught his students critical thinking and why would the Demon Research Initiative and The Initiative be in any way linked?
I said that I was a teacher too and my critical thinking skills told me that they could have used any word in the world, they used 'initiative' so the fans would link them with the ones in Buffy. It was an easter egg. But it was now also canon that the government and military knew about vampires since at least the 40s.
He also demanded to know why I didn't write that the mayor had been stopping The Initiative from being in Sunnydale before s3 and they were only there now because he was dead ... I was just ... BECAUSE THAT'S YOUR HEAD CANON.
It's a good enough theory, but there's nothing in the show to actually support it as being true.
I just ... sometimes silence is actually preferable, you know?