It appears that in some cases Google has been overzealous, hitting (temporarily) even websites with rather natural link profiles. In other cases, Google admitted algorithmic classification mistakes, publishing apologetic messages like this one Matt Cutts posted on Google+.
According to Patrick Altoft (branded3) the sites that have received unnatural links notifications fall under five main categories, and they are not just the ones participating in link exchanges or other types of link networks.
Your titles need to be authentic. They need to sound real. They need to sound like a human being wrote them that was not intending necessarily simply to rank for phrase after phrase.
Bad: It sounds like all you're trying to do is rank for keywords, especially if this is your home page or those kinds of things. Think about whether a normal human being would read that title and think, oh yeah, that sounds legitimate.