This bugs seems to occur when 2 pistons are un-powered at the same time, and is probably related to the new way in which pistons update. I have 2 example set ups in the images - a vertical and a horizontal one. In both you can see that when the piston extenders become deactivated the block which was pushed by the pistons is left alone.
MC-11269
Instantly retracting pistons cause blocks to be left behind
Resolved
Duplicate
0
Brett Parshall
(Unassigned)
Unconfirmed
Snapshot 13w10b, Minecraft 1.5
Retrieved
Duplicate of MC-9342, please use the search function to see if your bug has already been submitted. Currently over 53% of tickets are being closed as duplicate.