Most nastiest thing I've done is accidently licked my girls asshole while going down on her. The way I had her lips spread kind of pulled her anus up and when I licked up and down I eventually noticed what else I was licking. Most nastiest thing I've ever done knowingly was probably sticking my fingers into my Ex's ass and trying to (not successfully) stick my dick in her ass.
Most nastiest girl I ever messed around with, had my fingers inside her for a little while playing with her and up until 3 days later my fingers still smelt horrible. I washed my fingers with all kinds of different soap and nothing worked except for time.