I've seen premines stolen. I can't recall which coin, but it caused a shitstorm. And of course, proved the dev was a noob and the coin was crap.
I also remember that coin (not its name though...)
Actually with x11 coins, I often ended with my own fork and the premine... but it never sticked
actually I don't think it is really possible to steal the premine (well unless in that previous case... and noob dev
).
The dev just need to kill off the seed nodes and start again and call it the "correct" fork