Piston Head Retraction failture

I fired a sticky piston facing upwards with a 2 tick pulse, connected to a iron block, on top of the iron block was a piece of sand, and as the piston retracted, it left the piston head in the air.

Screenshot included in the attachments.

(The setup of the redstone was similar to one of the life-counters in sethbling's super craft bros map, and it was the middle piston that got stuck.)