Back in 2005 some car thieves in Malaysia tried to steal a Merc S Class with some kind of biometric immobilizer. When they realized they couldn't get the darn thing running without a finger print, they merely chopped the owner's finger off with a machete (I swear it's true: BBC Article).
What makes this sort of thing particularly nasty is that it doesn't have to actually work. The bad guys just have to think it might work, and goodbye finger. Or fingers, since they wouldn't necessarily know which finger was magic. Or eyes, if they get confused about what "Retina Display" is all about.