Any cliches that people use on the internet (and which will EARN you a post deletion, or worse, laughed off, any *serious* science forum) would have to include this favorite:
Correlation does not imply causation.
It's become so bad, (cliche-wise), being used by non-statisticians and non-scientists, that it is considered the domain of the "internet blowhard" who posts it simply to end all discussion..
It's considered the new and improved "invoking Godwin's Law" phrase.
(especially annoying to any ex smoker, as we all know Big
tobacco relied on dismissing correlational evidence to reject all links between
tobacco and lung problems and diseases).
The guys on science forums and blogs really are pretty funny about talking about it .... scathingly, actually.