During a showing of My Fair Lady in London, I was stuck next to a large Scottish man who sang along with all the songs. Behind me was a boy eating crinkly candy and his mom, explaining the whole plot. The Scot redeemed himself by turning to the kid and growling, "What the hell do you think you're doing?" The child was silent for the rest of the show.
Rivaling that was a showing of Les Miz where the man next to me sang along with all the songs, but made up his own words. He also tapped his feet, which shook the seats, and kept putting his jacket over the mezzanine railing, which lead to a near-altercation with an usher.
The final example would be a woman at Phantom in front of me, who felt the need to take her three sweaters off and put them behind her back, then put them on again. Then she started doing Yoga in her seat.
'"Contrairiwise," continued Tweedledee, "if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic."'
~Lewis Carroll