Subject: examples of advances due to fortuitous observations?

I'm collecting examples of important scientific advances resulting from somebody 
simply noticing an odd event. Obvious examples would be Galileo & the swinging 
lantern, Flemming & his contaminated Petri dishes, Rumford noticing the heat from 
cannon boring, Glaser noticing the occasional odd behavior of bubbles in a glass 
of beer. I would be grateful to learn of other examples.

Thanks,

Jim

Re: examples of advances due to fortuitous observations?