I don't remember anyone laughing in TWOK...except where it was intended.
I guess I didn't see it in a theater with a psychotic audience...
How about the chestburster in ALIEN? Geezus the theater I saw that at (century 22, the 3-screen cinema a couple hundred yards away from Century 23) friggin' near exploded with laughter when the thing came out of Hurt's chest. I didn't have that reaction myself, but I've thought of it often as an example of an audience not knowing how to react and just going nuts for a second in the wrong direction.