The spider gears have to to be placed directly opposite each other between the side gears, then rolled into position. I don't think there's room for one to be dislodged if they were lined up properly with the pin. The assembler just plain messed up installing them. I don't think there's much room for the loose spider to lay in the bottom of the case. It'll be sucked in between the ring and pinion in pretty short order.
