This is the point where a relatively serious person suddenly goes over the edge to a laughing person. It can be brought on by a joke or a situation, and it doesn't require any prompting although previous jokes increase the chance of breaking the laugh limit.
John was being very serious about his financial situation until he saw a very funny political video, and then suddenly he broke the laugh limit. Then he was laughing so hard that he almost cried.
by Chris Doc October 19, 2009