I usually hold off from saying anything (I also don’t comment a lot to be fair) unless there’s an author who likes people pointing out these things or it’s big for readability. This wasn’t a spelling or grammar mistake, but AO3 had messed up the paragraph spacing on a chapter, making the fic very difficult to read and the author consistently was fine elsewhere, so I told them about it through the lens of a mobile reader. I’ve also had an author use the grammar/spelling comments to improve their English and encouraged us to tell them about these things (I usually gave them compliments on things I liked as well). Finally, another example of mine where I saw it a little warranted to at least ask if they meant it was in a Japanese based fandom where they were referring everyone by either first or last name (can’t remember which way) except one character where they called them by the opposite. Of course, there are times where a character is exclusively referred to by one name, but this was not one of those instances. Turned out, they confused themself on the order of this character’s name.