I was useing a 3x3 piston door i made with sand in the middle, and then the sand block broke. Then I went to see why it broke, and it appears that the sand falls faster than the piston can retract the block it's holding. so the sand acts like it's falling on a half-block and breaks. 

     You can replicate this by powering an upward facing sticky piston with a block and sand (or any block that is able to fall) in that order. Then unpowering the piston.

     Me and my door would appreciate it if you guys could fix this not so game-breaking but still quite annoying bug