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)