Any chance you slightly pulled the trigger - relaxed - then pulled again, skipping that round and landing on a previously discharged round? I just easily - and repeatedly - duplicated this on my 632 Pro - with it empty. To verify this, make a pencil mark on the empty cylinder at a chamber. Line that chamber up to be 'next', then pull the trigger enough to start the clinder to rotate. Release the trigger. Fully squeeze the trigger - and watch that marked chamber sail past! That would be normal behavior. Obviously, don't partially pull the trigger!
A ftf cartridge will have a small indention in the primer - not unmarked or deep primer dent. When the fp hits the primer, it dents it slightly - just enough to light it off. The discharge recoil slams the cartridge back against the frame's recoil shield, driving the fp deeper into the previously formed primer indention, making the deep dent commonly seen in spent/empty cases. Aditionally, the primer will unseat from it's normal position slightly below the rim to even with the rim - from the case pressure. If the fp hit doesn't light off the primer, a slight dent will be left in the unfired cartridge. It's not unusual for a subseqent hit to light it off - unless it's a dud (ie, dud primer). If you have a propellant-less dud, the primer discharging will generally unseat the bullet enugh to tie up the cylinder.
I would also look carefully under the ejector star for manufacturing crud, carbon, cotton cleaning swipe thread, or even a brass wire bristle from a cleaning brush. It doesn't take much to set the moonclipped round away trom the cylinder, making some of the struck fp's energy be used to push the cartridge/moonclip forward - yielding a ftf. I've had this happen on two 625's - making me cognizant of that fact - and thus keeping a watchful eye for such with my 627's and Governor. Again, a slightly struck primer will have a teeny indentation. You said your cartridge was clean - it had to be skipped.
It isn't my revolver... but if it were, I would shoot a box or two of mild/inexpensive .38 plinkers - moonclipped - at one range trip. If no ftf's, I'd write it off to a 'skipped' round - and not use that return label. Of course, it's your revolver... Good luck!
Stainz
