Holy cow!
This could happen easily if you tended to carry one in the pipe. Every time you cleaned, or even cleared that gun, you would pull the magazine and eject the round. Then, when you take the weapon again, you would put the same round onto the magazine and chamber it.
I guess this is saying DON'T DO THAT.
It seems too easy to create this condition in normal usage. They may want to do something to the architecture of that round to prevent the bullet from seating deeper.
So what is one supposed to do with a live round after ejecting it?