Something I see all the time from both my fellow webmasters and from search engine optimizers is the problem of confusing correlation with causation.
Frankly, it drives me nuts.
Let me explain.
This is also known as the “cum hoc fallacy.” Basically, it’s the assumption that because two things happen together, one of them must have caused the other.
So how does this relate to search engine optimization?
Well, part of the problem for SEOs (and their clients) is that the process and criteria the search engines use to rank pages is pretty much a “black box” to those of us standing on the outside. By that I mean, we can try to figure out how they do it, we may even feel we’ve got a pretty good handle on at least some of the factors, but we don’t really know everything. Not for sure. Not all the details. (And, you know, the devil often lives in those details.)
Add to that the fact that the search engines themselves are....
SEO Insanity : Confusing Correlation With Causation | SEO Scoop