There are subtle shady things that happen during nearly every NFL play. Stepping on another player on "accident," getting a little extra shot in near the end of the play or whatever happens at the bottom of a pile. Then, there is double-leg kicking an opponent right in the chest in front of everyone.
San Diego's Corey Liuget didn't care about being subtle, he just straight drilled Oakland's Donald Penn. Not only did he obviously kick him right in the chest, but he wasn't flagged for doing it. Penn, however, did draw a flag for responding.
Let's recap: Penn gave Liuget a small shove which prompted Liuget to haul off and kick Penn in the chest. Penn was then flagged for responding to getting kicked.