rainborick - 4:09 am on Sep 13, 2013 (gmt 0)
It seems pretty straightforward to me. The initial message warns a webmaster that Google has detected a pattern of unnatural links pointing to his site. Although the message goes out of its way to avoid explicitly accusing the webmaster of intentionally violating the guidelines (ie. "If you don't control the links pointing to your site, no action is required on your part."), it is clear that they are giving the webmaster a chance to check the links and take action to remove or disavow any suspicious/bad links he finds before the final judgement is made to penalize the site.
If the "probationary" period ends and Google sees no change, they penalize the site if they believe the violations were intentional and the webmaster receives the second message that informs him of the penalty and explains how to resolve the situation.
A common response to the first message is, "I've done nothing wrong, so I don't need to do anything." Clearly, that is a risky attitude. Google's system and the quality raters are hardly omniscient. So, it would seem to me that a webmaster should take a close look at their incoming links when they receive the first message, and take some action to remove or disavow links that might be deemed artificial. And you should document those actions so you can demonstrate your intention to comply with the guidelines because if you do end up being penalized, filing a Reconsideration Request armed only with a claim of innocence has proved to be futile.