I've had the same happen to me with my revolver (round staying in the cylinder... not an AD/ND).
I don't have a CCH permit, so I open carry. I was getting ready to enter the city limits where open carry is prohibited, so I stopped in a parking lot to unload my revolver. I unlatched the cylinder, and dumped the cartridges in my hand and put them in my jacket pocket. I then put my revolver into it's case in my console.
Leaving the city I went to load up, unlatched the cylinder, and there was a cartridge still in there! Lesson learned; always press the ejector rod unloading the revolver, count cartridges, and verify cylinder is clear.