Comment by com2kid

My team (Microsoft Band) discovered the reason why the surface's keyboard sometimes wouldn't work when connected. There was a hardware bug in the cortex MCU the keyboard used involving waking from deep sleep. One of our FW engineers spent several months figuring it out and eventually reported it to the manufacturer, and to the Surface team. IIRC it was something about wake on interrupt in a specific deep sleep mode and also something around timing.

It was a rather nasty bug. Firmware is full of nightmare scenarios like that.