For what its worth, the firmware bugs are definitely not in my head.
I've just figured out what the mysterious lockup is.
If you leave the unit in the phantom "cooldown mode", and then switch it off, when it powers on, it's still in the cooldown mode, but doesn't indicate it anywhere. You have to blindly press + and - keys, with no indication that you've really done anything besides all the controls becoming unlocked again.
Someone, somewhere, forgot an if/then on power-on.