Hearing the story, you'd think he got mad when his NCAA Tournament bracket went bust or something, but we now have more details, from Adam Schefter of NFL Network, via Pro Football Talk. Marshall says he slipped on a McDonald's bag, stuck out his hand to brace his fall, and stuck it right through a TV. He needed a number of stitches and his arm is now in a cast, but he apparently will be fine long before training camp opens.
It's hard not to be at least a little bit skeptical of such a story, seeing as it sounds so strange, and seeing as many athletes in the past have lied about how they suffered injuries because certain activities are specifically prohibited in players' contracts. For now, there's no specific reason to doubt Marshall's story. But it sure sounds hard to believe.
UPDATE: Broncos' Brandon Marshall Admits McDonald's Wrapper Did Not Cause Arm Injury