Minecarts that are on a rail that gets pushed horizontally by a piston (regular or sticky) get pushed one block further than the rail it is on is pushed.

I noticed this in 1.9 pre-release 1. Testing shows that this does not occur in 1.8.9. I have not tested any of the 1.9 snapshots.

In the attached image, the close piston has not been activated yet. The other two pistons have already been activated, showing the result of a sticky piston and a normal piston. As you can see, the minecart has been pushed further than the rail was pushed by the piston. The minecart was just instantly moved to that position when the piston was activated, and no minecart rolling sound was heard.