[HartleyMBaldwin]

I don’t think the volume and pressure of gas escaping from a pierced primer would be sufficient to drive the hammer back fully against a full-strength spring (which would be necessary to pierce the next primer) and rotate the cylinder as well. Also, the primer itself would probably be pushed out of the pocket and jam the cylinder rotation. It sounds like a pretty far-fetched what-if to me, and I doubt that anyone could even deliberately rig a revolver to do that.

[Who is John Galt?]

It would definitely take a big hole in the primer (a lot bigger than your common 'pierced primer' event), but that's not impossible with a badly damaged firing pin. With a big enough hole in the primer, the primer cup might not set back and lock up the action. And if there's a missing bushing and weak hammer spring, who knows? I sure wouldn't want to attempt anything like that on purpose, or experience it by accident.

I guess the main thing to consider is, we all find it easy to picture properly functioning firearms, because that's generally what we own and shoot. But malfunctions do occur - including (on very rare occasions) guns firing with nothing touching the trigger (ask me how I know ;>)...