I work in an academic medical center in a support role (translation: I'm not a medical professional, but I hang around a lot of them as part of my job), and because of that I get to listen in on a lot of lectures on medical topics.
The subject doesn't come up much, as you might imagine, but from what little exposure I've had to the subject a penile "fracture"* is no laughing matter. It's incredibly painful when it happens, it requires immediate surgical intervention to repair, and if things don't go well you may not be able to do IT (as our brother Titty Mac would put it) again at all, much less do IT good.
I've seen pictures both of what a penile fracture looks like from the outside when it happens, and of what it looks like on the inside as they're cutting a dick open to repair it. If you're feeling especially brave, do a Google image search on "penile fracture". But I'll warn you ahead of time, you'll probably regret it. To give you a taste, this:
is probably the least sick-making result from that search.
Thankfully, it's not an injury that is at all common, but if it ever happens to you, you're in for a time that you'll never wish on your worst enemy.
ETA: Here's a first person account of breaking your dick:
Broken Penis. It may make you wince, but not as much as some of those pictures will.
Cheers,
bcg
*In spite of the widespread use of the term "boner" to describe an erection, strictly speaking a penis can't fracture, since there ain't no bone down there. More accurately it should be called a "penile rupture", since the injury is basically a rupture of the tunica albuginea, a fibrous sheath that surrounds the corpora cavernosa and corpus spongiosum penis, which taken all together (and wildly oversimplifying here) are the things which fill up with blood and make the boner resemble the bone from which it takes its name. And it's all that blood spilling out of the rupture and into areas it was never meant to be which makes the vast majority of the results of the Google image search I mentioned above so sick-making.