"In the long run we are all dead." - John Maynard Keynes
The irony in Keynes' quote is not just that it is so often misinterpreted, but that it is generally misused in a way to defend a point that is the complete opposite of what Keynes was trying to say. The full quote (and description) goes as follows:
"The long run is a misleading guide to current affairs. In the long run we are all dead. Economists set themselves too easy, too useless a task if in tempestuous seasons they can only tell us that when the storm is past the ocean is flat again."
-- A Tract on Monetary Reform (1923) Ch. 3; many have thought this meant Keynes supported short terms gains against long term economic performance, but he was actually criticizing the belief that inflation would acceptably control itself without government intervention.
-- A quote attributed to John Maynard Keynes, as a catch phrase in his rebuttal to laissez faire economics, as proposed by Adam Smith. The idea is that rather than "in the long run, everything will even out", that "in the long run, we're all dead", so we should aggressively push to rectify economic problems (within our lifetimes), rather than just letting everything work itself out.