This was quite annoying, especially since I failed at googling a cure for it.
What happens: whenever you type “'t” in a comment on a Java file in Idea followed by a whitespace, it gets replaced with “<></>“, and whatever you type after that is added as if it were an HTML tag pair.
If you actually pause after typing “'t” (which I don’t know why it’s something I never did until now), you’ll notice IntelliJ’s auto-complete popup telling you what’s going on:
It turns out that “t” is the shortcut for “tag pair”, which might be awesome for HTML coding, but it’s not as nice when you’re writing comments.
To disable that, go to Settings/Editor/Live Templates/, expand the html/xml line, and uncheck t (tag pair). Hit OK, and it’s gone!