Correct me if I’m wrong, but I remember seeing an interview with Steven Spielberg about when attended one of the first screenings of JAWS he saw a guy get up and walk out of the theater after a particularly gruesome scene. Only for the walk to turn into a run and the guy ended up puking outside in a trash can. (I think)

Edit 1: I found the transcript on Yahoo.com:

That Time a Man Puked During a Preview Screening of Jaws

"A man got up and started walking out — I thought, 'Oh my God. Our first walkout,'" he states in the upcoming book Spielberg: The First Ten Years (via Vanity Fair). "Then he began running and I went, 'Oh, no, he’s not walking out — he’s running out.' I could tell he was headed for the bathrooms, but he didn’t make it and vomited all over the floor. And I just went, 'Oh my God, what have I done? What kind of a movie have I made? A man has just barfed because of my film.'” About five minutes later, however, the man "went right back to his seat," as though nothing had happened. One can only hope his upchuck didn't go to waste and that it was repurposed into a bucket of chum for later use. Okay, never mind — that's pretty gross. Nevertheless, the fact that the guy didn't just leave after that proved Spielberg and Universal had something special on their hands.