I'm not endorsing his advice -- not that my endorsement would be worth anything. If I were you, I would not seek endorsements in this field from a 41-year-old journalist who has been studying the martial arts for less than two years.
That said, I do think I can back up my claim that you really want to avoid hyperextending your elbow while punching a bag.
I can also say with authority -- I will really stand by this -- that if you're used to hitting things while wearing heavy gloves, you need to be really careful to remember that if you're not wearing them, punching something hard can hurt your hand quite unpleasantly. After today's further misadventures, I can be confident I'm not steering you wrong with that counsel.
(I see no need to illustrate the consequences of failing to heed me on that with a photo; I'm sure your imagination will suffice. Just c.f. earlier training error photos and imagine the same phenomenon, but involving fingers.)
I guess I'd actually be a reasonable authority to consult if you're interested in learning about any kind of dumb training mishap that would probably only happen to you if you were quite clumsy, like ripping off the skin of your wrist on the flapping velcro strap of a grappling glove, which also happened to me today.
But if you want the kind of martial-arts advice you'd get from someone who has actually spent a lot of time subduing extremely violent prison inmates, and apparently has had many experiences along the lines of "looking into an empty fresh skull and the smell of brains," you might try Rory Miller.
He sounds quite a bit more qualified than me.