The myth

A 19th-century Member of Parliament predicted that, given the rate of growth of traffic, London would be six feet deep in horse manure by 1910.

The "truth"

The details vary with almost every telling: the doom-merchant is a politician, scientist, city planner, or journalist; the doomed town is London, or various cities in the USA, or a particular famous street such as the Strand; the predicted date of the catastrophe ranges from the turn of the century to 1950; and the depth of the dung goes from knee-level upwards. The underlying message, though, is consistent: forecasting through extrapolation is risky, because it can't take account of technological revolutions.