When I saw Thoroughly Modern Millie, during the "Millie seduces Trevor Graydon scene," Chris Sieber broke out of character for a moment and looked at the audience a couple of times, trying to hold in a laugh.
In the same performance, Angela "Miss Dorothy" dropped her purse.
During a performance of "Good Morning, Baltimore" on the Regis and Kelly Show, when Marissa sang, "But my feet tell me go!", her purse slipped off her shoulder and she quickly had to pull it back on her shoulder.
During the 42nd Street tour, during one of the dialogue scenes, one of the characters was standing off to the side (he was at the piano, I think he was the one who played it). I noticed that a piece of fabric or a feather began to *slowly* fall from the side towards the stage. Anyways, the guy at the piano eventually caught it, brought it backstage, and came back out.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611