If I remember correctly (which I sometimes do), there was an episode called No Laughing (or something similar). The plot was, neither Beavis nor Butthead were allowed to laugh and everything that could possibly make them crack up happened. The last five minutes of the cartoon is just them laughing.
I may have the episode name wrong. I may have made this whole thing up or it may be a bad dream. If this episode does exist, is it on one of the laser disc collections? And if so, which one?