Short story: An instructor at my LDS got hurt when an OW student panicked and ascended rapidly while sharing air.
Longer story:
'*The instructor was diving long hose, primary donate.
*Student got panicky for no good reason, and asked to share air.
*Instructor started to hand off the primary.
*Student grabbed primary and bolted, pulling the instructor by the neck. (The long hose still had a turn around his neck.)
*Instructor ended up out of action for several weeks/months until his neck mobility recovered.
I've got no qualms with sharing air with a panicked diver, but I don't use longhose with students. It's the same rig they use, so we're all familiar with each others' gear. In this case, if the hose wasn't around his neck, I'm pretty sure the panicked student would have lost the 2nd stage as he went up. I can't imagine a panicked person gripping a reg in their mouth so tightly that they could pull up an unwilling buddy. Seems far more common for panicky divers to REJECT the regulator rather than keep a death grip on it.