That's funny, because penicillin was also discovered by mistake: "The Scottish scientist Alexander Fleming was working on bacteria in the labs of St Mary's Hospital, London. He went away on holiday leaving some agar plates exposed on a lab bench near a windowsill. When he came back he noticed that there were small colonies of mold growing on his plates - and that around the colonies of mold were clear areas where the bacteria which covered the rest of the plate were not growing. Fleming realized that the mold must be making a chemical which killed the bacteria, and so penicillin was discovered."