Kaspar Szymanski, a Googler responsible for Webmaster Communication and spam prevention, said in a Google+ post that "correlation does not imply causation."
Something many have said before, but heck, it can't hurt to say again. It comes off a lot stronger anyway when a Googler who sees the details of a specific situation says it himself.
Browsing Google webmaster forums it seems like some folks confuse correlation with causation. That one link from a dodgy directory you got last week or changing H2 to H3 yesterday did likely not contribute to your sites visibility today. Please keep in mind that correlation does not imply causation. Rather than assuming impact, check your Webmaster Tools message center or better yet enable message forwarding to email :)
He is clearly hinting that someone in the forums blamed a ranking drop to a header tag change when in reality, Google emailed the webmaster via the Webmaster Tools message center the real reason. If he just checked there, he would not have felt his H2 change caused such a serious ranking drop because it did not.
Often it is hard for SEOs to realize that Correlation Does Not Imply Causation but they need to. Too many variables in search to simply pinpoint one change as the deciding factor in many cases.
Forum discussion at Google+.
Image credit to ShutterStock for SEO graphic.