Comment 1 for bug 46445

Further investigation showed that this only appears with the original iMac keyboard, which has a power button. So this is obviously not solely an ubuntu problem, but an unfortunate combination of bad timings during shutdown. Somehow the keyboards 'thinks', its power button is pressed again, and restarts the machine.
*BUT* this doesn't happen with the Apple OS's on that machine, so I#d still call it a bug