I didn't know it was a thing until I read about it on The Blue: Corpsing.
That would be what happens when you are a cast member and one of your fellow players forces you to break character and laugh. As defined: "Corpsing is a British theatrical slang word for when an actor unintentionally breaks character during a scene by laughing or by causing another cast member to laugh."
A compilation of these instances on Saturday Night Live has recently been circulated.
However, I say that an earlier generation of sketch actors makes these guys look like pikers when it comes to corpsing. Here's an example.
Apparently this incident was pretty legendary. Vicki Lawrence tells the story: