There have been 45 presidential elections since 1828. In at least five, the race went to the second-most-popular candidate because of a spoiler. That's an 11 percent rate of catastrophic failure. Were the plurality vote a car or an airliner, it would be recognized for what it is – a defective consumer product, unsafe at any speed.
— William Poundstone: Gaming The Vote, Hill & Wang 2008 (page 91).
Actually, if you look at a larger (worldwide not merely USA) dataset, then it appears "11 percent" is an underestimate.