After inoculating several dishes with the bacterium staphylococcus, Alexander Fleming forgot to cover them up before going on holiday. On his return, one of the dishes had grown mould. Fleming observed that the bacteria around the mould were all dead, thus discovering that the mould Penicillium had antibacterial properties.
In 1492 Christopher Columbus set sail westwards intent on discovering a new route to Asia. Miscalculating the Earth's circumference meant he actually landed in America, opening up trade links between the old and new worlds.
In 1839 Charles Goodyear accidentally dropped some india rubber mixed with sulphur on to a hot stove, discovering the vulcanisation process which made possible the commercial use of rubber.
Chef George Crum invented crisps in 1853 when a customer at a restaurant in New York, returned his fries to the kitchen. In anger, Crum sliced them as thinly as possible, over-fried them and doused them in salt. The customer was delighted and the crisp was born.