I once tickled a girl I find very hot while in her boyfriends bedroom while her boyfriend was there.
We were all watching (there were four of us) a Robin Williams stand up comedy on his PC, and I just tickled her for no reason, and she yelped, then she tried to do it back, so I got very Dom and said 'you think so do you?', pinned one arm beneath my shoulder, held the other in my hand, and tickled her with my free hand for about a minute. Then I asked her 'did you learn your lesson?' and she gave me the finger, so I replied 'Oh really?' and hooked one of her feet under my arm and went to work.
My other friend found it funny, and her boyfriend just got quietly annoyed. Even so, I felt that it took some daring...