Remember when that guy got his penis stuck in a weight plate and firefighters had to come saw it off him? Apparently, it wasn't a one-of-a-kind accident, because there’s an actual name for doing that.
So how dangerous is it? Believe it or not, jamming your dick into a small hole could possibly be even worse than breaking it. If you stick a flaccid or semi-erect penis into a tight opening, you might not be able to remove it after it becomes erect; if this happens, new blood with fresh oxygen won't get through. With a lack of circulation in your penis, you could develop erectile dysfunction and long-term pain while urinating. Or worse: Your penis could actually turn black or blue and the tissue could die, which means penile amputation is likely to occur.
The good thing is, it's easy to prevent. Just don't put your penis in any constricting objects, including too-tight cock rings. It’s better to be safe than sorry when it comes to exploring sexual fantasies.
In the wise words of the German firefighters who spent three hours cutting the weight plate off the aforementioned guy's dick, "Don't imitate such actions!"