“Post hoc ergo propter hoc.”
Which, directly translated is:
“After this, therefore because of this.”
It suggests, often (but not always) incorrectly, that if Event B follows Event A, then Event A caused B.
This is also known as the Post Hoc Fallacy, and it is the most common flaw of logic I see online. Heck, it’s certainly the one that irritates me the most online, especially within the healthcare domain.
Causality of anything is a tricky subject (up to and including the question of whether or not causality even exists), so we should do our best to not make Post Hoc Fallacies, the lowest-hanging fruit of logical mistakes.