according to the spec as i understand it, the bulk-update commands that operate on 8x8 blocks can pass along x,y offsets that aren't actually multiples of 8. Right now, untz_monome will break if it sees those, leaving the remaining serial bytes specifying the data left in the buffer.
So, any 'break' in that switch statement can potentially leave junk in the incoming serial stream to foul up the next command.
this is based on my reading of http://monome.org/docs/serial.txt, which seems a little obtuse, but hey.