Tampa Bay's Simon Gagne has a history of concussions. What has become obvious is that players who have suffered past concussions become more susceptible. Gagne takes a hard hit from Washington's Scott Hannan and it appears has suffered another head injury. This is a clean hit and definitely not a hit to the head. In fact I have watched the video several times and don't even see his head hit the ice. Yet Gagne clearly looks like he's in trouble.
This is why the notion of legal and illegal head hits makes no sense. By putting players in a position where they can suffer a concussion you are pretty much shortening their career. As you can see from the Gagne hit at a certain point it doesn't even take a blow to the head to bring back symptoms.