Well their work is not exactly brain surgery, with lives on the line hanging in the balance, but that always looks unprofessional to me and not as funny as it otherwise would be.

Nothing so epidemic as laughter. :D
http://www.youtube.com/watch?v=Z4Y4keqTV6w

When done sparingly, comic actors losing it can be a real blast. Unfortunately SNL had a period where a couple of the actors notablely Horatio Sanz and Jimmy Fallon could hardly stay in character for a few minutes at a time. That was not good tv.