I've seen that a couple of times last year, and I use OS 2006.

I rarely turn it off, but on both occasions I was on flights and wanted to save batteries for video playing. I turned it off before going through security, but once on the flight it was dead. I suspected a crash, but removing and replacing the battery didn't help.

Fanciful theories about the x-ray machines aside, I rarely turn it off usually, so maybe it locks up somehow during shutdown.