The number of Broken Arrows is amazing, really. When I was working on such things I had some research on the topic and it is amazing. Most of the basic broken arrow stuff is UNCLASS, so google away. I imagine there is a lot of info out there.
I want to say there are like 5 still unaccounted for that were accidental launches. Can't remember. (ok, I did a quick google and it said 6.)
A modern PU-239 implosion trigger (what our "h-bombs" are) is very unlikely to reach critical mass (i.e. KA-BOOM) from dropping off a bomber vs. being launched intentionally.
However....I sure as hell don't want B-52s flying around with live nukes that they don't even know they have no matter how unlikely an accidental nuclear incident is!
So all that said, this is really a disgrace to the USAF. We blast Russia for poor nuke security, then this happens.