They always do that! Its a funny show you cant expect them not to laugh! Personally i think it makes it funnier when they do that! So i really wouldn't consider it a slip-up!
Apparently some of you have never seen "Carol Burnett Show" Harvey Korman and Tim Conway were always laughing and at times Carol would too. It happens more than people think. Just about every comedy I have watched I have seen people laugh or trying to hide their laughter. Sorry no slip up.