Andrew Gelman has an item in his handy statistical lexicon that he calls “The ‘All Else Equal’ Fallacy,” summarized as:

Assuming that everything else is held constant, even when it’s not gonna be.

Since that idea that “all else is (or would be) equal” is often represented by the Latin ceteris paribus, and the point of Gelman’s fallacy is that (in cases where it applies) the assumption is simply ridiculous (parodic), perhaps we can humorously term the fallacy (in abused pseudo-Latin) “ceteris parodis.”