During the filming of Rocky IV, director-star Sylvester Stallone thought it would be a good idea (for realism) to do a shot where Dolph Lundgren actually punched him as hard as he could.
The next thing he knew, Stallone was spending four days in intensive care with a bruised heart.
He then had to convince his skeptical insurance company that the injury had, in fact, been caused by a punch and not (as is usually the case with that sort of injury) from being hit by a truck. He finally showed them the footage and exclaimed, “Dolph is a truck!”
Stallone later admitted that this had not been a good idea.