Question: Has anything unintended ever happened during an experiment due to careless-ness or not paying enough attention?

Katharine Schofield answered on 21 Nov 2011:

Plenty of things have been discovered by accident – the discovery of radioactivity was one example. The French physicist Henri Bequerel was doing some experiments on minerals. He left the mineral (potassium uranyl sulfate) close to some photographic plates, and found that simply by doing that, an image of the mineral could be seen on the plates. He realised that this was because the substance was emitting radiation (i.e. it was radioactive). He won the Nobel prize, shared with Pierre and Marie Curie, for this discovery.