I remember reading a story on here one time where the girl ended up naked in front of a guy and his group of friends, and then decided she wanted to give everyone a show so she started fingering herself in front of everyone.
That or maybe just your girlfriend likes to "accidentally" show too much when she bends over in a skirt, or answers the door less than fully clothed for a delivery guy, or just any form of showing off to people to give them a show.
I can't find the story that I was referring to, but here's another one that's a really good example of what I'm talking about.
http://pastebin.com/iYMfZt8y (relevant part starts at the third paragraph)
Source: reddit.com/r/gonewildstories/comments/38ems8/guys_any_stories_where_you_let_your_gfwife_put_on
I’ve posted about this here in the past, but since you asked… After losing a bet to me, my ex-GF flashed the pizza guy. We both found we got off on it so she ended up giving ever more daring "tips" to the (same) pizza guy over the ensuing couple of months. The lucky kid eventually got to see her fully naked and even copped a couple of gropes. Ultimately, he got a hand job from her and came all over her DD tits. She also went topless at the beach with a mate of mine along (specifically) for the show. She also wanted to give us both a hand job simultaneously on the beach but circumstances didn’t allow it.