The Dallas Cowboys and Washington Redskins were the only two teams that decided to take (obvious) advantage of the fact that last season was played without a salary cap. Because the Redskins and Cowboys treated the uncapped year like what it was by frontloading the living hell out of some contracts, they got hit with a big cap penalty. They're none too happy about this and they're filing a grievance against the league and the NFLPA.
Mike Florio has the details over at Pro Football Talk. This is the most important part of the grievance and PFT's article on the situation.
The fact that the Cowboys and Redskins included the union in the grievance triggered the procedure that allows the teams to avoid a situation in which Commissioner Goodell resolves the matter. Instead, a true outsider will determine whether the action complied with the terms and/or the spirit of the labor deal.
This is a big deal, because Goodell would probably just tell them to go away and put his foot down. Instead, the final ruling is a little bit more likely to be fair and reasonable.
As it stands, the Packers will get a couple million in extra cap space because of the penalties against the Cowboys and Redskins.