A lot of people here have a fair bit of scientific background so I thought it could be fun to get the stories. Not sure if this shouldn't be in Off-Topic section so please feel free to move me. Anyway, any scientifically reasonable experiment (the 'placing a drunk friend's hand in a bowl of warm water while they sleep' doesn't count) in any scientific discipline counts.
Anyway, mine is an optics experiment during my second semester of 1st year college. We were finding the focal length of a concave lens by propping it up to a convex lens of a known radius of curvature and using the Lens Maker's formula. It seem to be grand enough mostly until I did some calculations with the said Lens Maker's formula and got a focal length of more than a kilometer!!! I must admit I found it pretty funny, since imagine trying to find a focal length of this size using the approximate method of focusing distant object on a screen. I could just picture it:"Right, you start running that way with the screen until I look really small!"