In theory, there may be a cook-off, where the heat from the bottom round generates enough heat, long enough, to cause the primer in the cartridge above it to fire and so on. I suspect the more likely event would be that by the time the energy of the bottom bullet was transferred to the top unfired cartridge, there would be little “oomph” left, possibly enough to cause hurt, but not lethal. Even more likely is that the tube would explode because the expanding gas and bullet couldn’t push the cartridges on top out quick enough.