when you have a BUD-piston facing up whit at least 2 blocks off air under it and than place a sand block at the bottom (or gravel/dragon egg) it will think that it gets pushed up and very very rarely duplicates.
MC-11419
BUD piston+sand
Resolved
Duplicate
0
RedJokerFTW
(Unassigned)
Unconfirmed
Minecraft 1.5
Retrieved
Duplicate of MC-11217.