This happens to me all the time. The worst was when I was watching Penny Dreadful and Dorian Grey has that 80 minute sex scene (not actually that long but it sure felt like it). Me mom walked in and thought I was watching porn.
Incredible movie, but goddamn, I can't watch a huge portion of the final act. The wife of actor David Gale--who played the eventually headless guy--divorced him shortly after the film was released. The cast mentions on the DVD commentary that it was highly suspected that his, uh, convincing performance in the above scene is very much to blame.