If sticky piston pushes a fence gate and then fence gate pushes the player - fence gate acts like a full block and pushes player (or entity) like normal (any other) full block.
I wanted to push player a half block only and that's how I found out this simple little bug.
Probably related to the fact pistons can't move blocks that contain NBT.
Also it would be hard to express such an effect whilst the block is being moved.