The Wilhelm Scream--I knew they did this sort of thing!!!
This article in boingboing discusses the uncovery of The Wilhelm Scream. Follow the link in the article to a vid clip that illustrates the use of the scream over decades of movie making.
This does not surprise me at all. In fact, I have often figured they did the same thing with canned laugh tracks: take some recorded, hysterical and remarkable laughs from other audiences and reuse them in layers to create the impression of an audience wetting their pants.
I even know some people (Ernie, you are the one I am talking about!!) who have laughs like that. I have even witnessed the phenomenon occurring within my own theatre where a person, usually a woman, laughs so distinctly and remarkably that the audience winds up laughing harder at her than at the comic action on stage. It helps if the person with the laugh also finds things funny at times that most in the audience do not, so that his/her laugh cuts through all other distractions.
I giggle just thinking about it.