It happens twice on my N8, can't respond on any touches.
The last one was happen after i flashing without cleaning/flashing the mass storage/emmc.
I guess it's some kind of apps conflicts on the mass storage.

Maybe you could use baem90 way. My N8 was succesfully flashed using Navifirm dead mode flashing, but i'm still don't get it how it would be on N9.