The last record-breaking accidental oil spill in the Gulf of Mexico was the Ixtoc I, in 1979. I should also note that the Atlantic Empress spill, also in 1979, and the Amoco Cadiz, the previous year, had a combined spillage equal to the Ixtoc I.

The record-breaking deliberate oil spill happened during the Gulf War, in 1991.

In the past thirty-five years, there have been ten oil spills in which over a million barrels of crude were released. I don’t see how you can classify this kind of thing as “low-probability”, in the same category as asteroid strikes.

http://en.wikipedia.org/wiki/Oil_spills#Largest_oil_spills