In this lawsuit, it is alleged that the NFL covered up evidence that concussions can cause long term damage. Check out a paragraph from this Slate article:

“This is smart pleading, made even more compelling by the complaint’s citation of one of the most astonishing statements, surely, that’s ever been made by anyone associated with a professional sport. In a 2006 article published in the journal Neurosurgical Focus, David Viano and Elliot Pellman—two members of the league’s concussion committee—summarized their “research” to date. The fact that most players returned to the game within a week after suffering a concussion, they explained, was proof that “mild [traumatic brain injuries] in professional football are not serious injuries.” It hardly need to be said that teams—and their employees, including doctors—faced strong incentives to encourage players to go back on the field, fully recovered or not.”