In one production of "Oklahoma!" I was in, the fog machines malfunctioned, sending all of their contents into the stage area at once. It was at the commencement of the dream ballet sequence and on the recorded video of the show, it looked something like an atomic bomb. The dancers tripped over eachother, the orchestra played at their own will- incorrect notes, the audience was choking, and our tech director was laughing her head off.
"Are you sorry for civilization? I am sorry for it too." ~Coast of Utopia: Shipwreck