Having uh, accidentally entered my duress pin a week or so ago I'll tell you now it works: Enter pin as normal, device starts to unlock, pin pad fades back then the device hangs for a few moments and reboots. Then it rebots to a menu that states the boot image is corrupted and your only option is a factory reset.

Point being is that it looks normal, unless you know what happened it could have been a memory issue and the phone just died. The GOS boot graphics give the game up though, I wish they'd change that especially with more scrutiny every day against devices running it.