From Act I of Travesties, by Tom Stoppard (the acts are not divided into scenes, but if you would like to look it up, this passage appears on page 36 of the 1975 Faber and Faber edition):
CARR (stiffly): I believe it is done to drink a glass of hock and seltzer before luncheon, and it is well done to drink it well before luncheon. I took to drinking hock and seltzer for my nerves at a time when nerves were fashionable in good society. This season it is trenchfoot, but I drink it regardless because I feel much better after it.
TZARA: You might have felt much better anyway.
CARR: No, no—post hock, propter hock.
TZARA: But, my dear Henry, causality is no longer fashionable owing to the war.
I am therefore tempted to suggest, in response to E Propter Hoc's call for a catchy slogan refuting this fallacy, "It could be the seltzer." But obscure allusions will only carry us so far. In practice, it seems that the closest thing to a popular anti-fallacy catchphrase is the rather literal-minded "Correlation is not causation"—which, if you want to get picky about it, is really directed against the closely related "Cum hoc ergo propter hoc" fallacy.
These seem like such obvious fallacies that, if one considers them in the abstract, it may be difficult to imagine how anyone falls into them. But people do, all the time. This happens quite frequently in situations in which the supposed cause and the supposed effect are really two effects of some third factor which has been overlooked.
Here is an example. Francesco Rutelli, the mayor of Rome, is quoted on page 57 of the May 2, 2005 issue of The New Yorker as saying:
I think cities are like languages. If a language doesn't change, grow, and evolve, it dies. It is the same with cities—a city must be transformed from time to time.
Rutelli knows (I hope) more about the survival of cities than I do, but his "Evolve or die" view of language is simply preposterous. The fact is, if a language has a sufficient number of speakers to ensure its continued existence, then it will inevitably continue to change. Little differences from idiolect to idiolect, shifting social pressures, contact with speakers of other languages, and the need to describe new discoveries will all exert their various influences on the language, eventually shaping it into something quite different from what it had been. On the other hand, if a language has so few speakers that it does not change, then it is almost certainly doomed, not because of its stasis, but because people find it less useful than languages that will enable them to communicate with larger numbers of fellow-speakers. So sure, "if a language doesn't change ... it dies," but that doesn't mean that you can save a dying language by fiddling with its syntax or revamping its vocabulary. In fact, that will do about as much good as drinking a glass of hock and seltzer.