When you push a minecart rail with a pistion and a minecart on top of it, the minecart does not consistently move with the rail.  Nor do entities such as boats stay on top of the same block when it is being moved.  This was not an issue in previous versions of the game.  As a result, the physics of minecarts have become very glitchy and inconsistent.  This is a huge problem for players that build moving redstone machines.