If a pack has not been properly cared for during it's life, such as equalizing all the cells before charging, the cells may start to perform at different levels so in a sense you're not longer running a matched pack. I'm not saying this is what you did and it is entirely possible that one cell did die, I've seen it happen and had it happen to me before. If the battery was already fairly discharged when you put the pack on the tray the dead cell may not be too far below par, I would try to charge the pack after equalizing it on someones Futaba CDR-5000 to see how much performance that cell has lost compared to the rest of the cells.
If the dead cell has only slightly less runtime you might not notice much of a difference, so if it's not completly dead the pack may still make a decent practice pack.