Some fashion crimes are so serious that they transcend even the normal rules of fancy dress, meaning that even although you’re dressed like that deliberately and for "fun", it’s still not OK, and you’re still a Fashion Criminal for it. For instance, it is never OK to wear your thong over your tights, and it’s even LESS "OK" to wear your thong over your tights when THAT’S ALL YOU’RE WEARING. Add a shaggy fur coat and some high-topped trainers to the mix, and we don’t care that it was a fancy dress party, you shouldn’t be out in public. No, not even if you are Naomi Campbell.
Also: the "thong-over-tights" thing? Isn’t that really uncomfortable?