(Also affects older snapshots of 1.9.)
Chicken jockeys do not have a zombie riding, and the console says "Received unknown passenger for vv['Chicken'/158909, l='MpServer', x=520.49, y=32.00, z=579.13]". Also confirmed on older releases of 1.9 snapshot (I don't know which), as I've had the chicken bug happen there too, but I cannot say if the console was popping up the messages.