Cross-dressing...maybe?
I remember a fic from several years ago, in which Snape either lost a bet or did a dare (I'm inclined to say it was a bet) with McGonagall. In short, he had to wear lace underwear for a certain length of time. We then progress to the Snarry. That is literally all I remember but if anyone knows it I'd like to read it again.
ETA: I also seem to remember that the lace moved in some manner. I can't remember how or why, but there was a definate instance of enchanted underpants.