When I play f8 it gets rejected I guess as it's autoqueening
Here it doesn't matter as there are multiple mates in one, but if a position with only knight promotion mate this would actually prevent me from posting anything)
(no I'm not suggesting to change the interface, but rather to filter out captchas with this issue)

21

That's good to be aware of, we'll make a note and try and address this at some point

31

That would make for a great lateral thinking puzzle "Some guy played a number of games on a chess site in a way to deliberately break all of the forums on the site, how?"

3