Cum Hoc, Propter Hoc
Because two or more events occur consecutively or simultaneously, one is not necessarily the outcome of the other. Sequence of events is not always the result of cause and effect. The swallows come to England in the spring, but do not bring the spring.
“[Free trade and revival of trade] says Lord Penzance, came simultaneously, but, he adds, `There is no more dangerous form of reasoning than the cum hoc, propter hoc. ' ” —Nineteenth Century, April, 1886.,
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894