Near the beginning, when the "Dudes" go to their psychic friend, at one point, I believe it's when he begins to chant, that you could see a little mosquito fly up his nose.
If you watch the DVD commentary....it is a fly that lands ON his nose and he swats it away. They say it happened in one of the many takes, at first it was not in the movie but in the final editing the director decided it was funnier with the fly so he put that take back in the movie. It is a real fly, and it doesn't go up his nose it lands on it.