One of the worst cliches is the claim, frequently though almost certainly incorrectly attributed to Albert Einstein, that “insanity is doing the same thing over and over again and expecting different results.” This is, of course, false and trivially false at that.
Whenever I hear people say something along those lines, I wonder what they expect to happen if they flip a coin. Say, they flip a coin once and it comes up heads. Do these folks expect all future coin flips to come up heads until the end of time?
In fact there are so many counterexamples to this particular claim, that its hard to understand how it ever became popular cliche (unless the attribution to Einstein was sufficient).
Update – May 2009: This phrase seems to originate in a 1983 novel by Rita Mae Brown. If anyone knows of an instance of the phrase appearing in print prior to 1983, I’d to know about it.