>>61611
It's even easier than that. Just a quick regex for >>(postnumber) and checking if postnumber group matches the thread you're replying to's post number. If there's a match you return an error advising not to do that without submitting the post. Alternative solution would be to post anyways, but strip the OP quotelink out, but that could cause confusion if someone is also replying to someone else.

>>61602
A place where you think you go to for anime lulz and click popular videos and now you're watching a mass shooting or nazi videos instead.