Laughter-triggered muscle weakness

Kay Underwood, 20, of Leicestershire, England, frequently falls over with laughter. Literally. She suffers from the medical condition cataplexy, which manifests as sudden muscle weakness that can be triggered by intense emotional experiences like anger, surprise, and laughter. She is also narcoleptic. From BBC News:
The University of Lincoln architecture student has in the past collapsed 40 times in a single day.

She said many people think that she is joking when her condition does make her fall over.

Ms Underwood was diagnosed as having cataplexy a year ago, but believes she has had it for about four or five years.

She said: "I think a lot of people, if I've told them about it and they've not seen it, would quite like to see me do it (collapse), so they try to make me laugh.
Laughter-triggered cataplexy (BBC News)