It seems that if you ask anyone they will say it has happened to them one time or another, and at different levels, i do believe agree with darkblade that it has to do with the set or setting and the mood that your in prior to dropping.
A couple of weeks ago at a recovery of all places in Melbourne i got so horny that i needed to go home and have a cold shower or i was going to fuck something and i mean something it could have been the couch i was sitting on.
I have also heard that a good way of getting over it is just to dance it off. I doubt it will happen everytime, but if it does just let me know where she will be next time, so i can help her through it.
Just because your paraniod doesn't mean your not being followed...............