During the filming of Rocky IV, Dolph Lundgren punched Sylvester Stallone so hard that he had to spend eight days in intensive care.

In an interview with Ain’t It Cool News, Sylvester Stallone was asked what his worst injury was. He explained the injury he had during Rocky IV as the worst, but also brought up an injury during Rocky II. 

Prior to the filming of Rocky II, Stallone had torn his chest muscles competing in a bench press competition. But that wasn’t anywhere near as bad as what happened with Rocky IV. 

Stallone felt that in the first round of the fight with Lundgren, the two characters should be fighting like “pit dogs,” and that none of it should be staged. He goes on in the interview to say that the first twenty seconds or so of the fight is real, but after a few takes of doing actual fighting, he felt a pain in his chest, which he ignored.

Later that night, he couldn’t breathe very well and so he went to the emergency room. Lundgren had punched his chest so hard that his heart hit his breastbone and had begun to swell. His blood pressure was higher than 200. It’s a good thing Stallone sought medical attention, because if he hadn’t, his heart would have continued to swell until it stopped. And then we wouldn't have The Expendables. 

(Source)