Uncontrolled physical access.
It's already a well-known truism that if the bad guy has physical access to your machine, it's not your machine any more.
It's therefore not all that surprising that if a bad guy has access to your machine, and then you have access to it, and then he has access to it again, you're twice fucked. Because basically the second time /you/ had it, it wasn't your machine that you were entering your password into any more, it was the bad guy's.