After the Lions made a stop on third-and-goal on the opening drive of the second half, defensive tackle Ndamukong Suh lost all composure and stomped a Packers player. He was flagged and ejected for the stomp. Not only did this result in the Lions losing one of their best players, but it also gave the Packers a fresh set of downs. They scored two plays later.
Here's a look at the incident (via @jose3030):
After Suh's ejection, the Packers went on to take a 24-0 lead. The Lions just scored to make it 24-8, but there's no doubt this incident will be considered the point in the game when the Lions lost all control. It was just a moronic thing for Suh to do, and you can bet he will be hearing from the NFL about a fine and possibly even a suspension.
I have vigorously defended Suh in the past when people have called him a dirty player, but there is no defense for stomping an opposing player and costing your team like that.