As seen in the screenshot and video below, placing a minecart track and pushing it with a piston, then retracting the piston within one tick does not properly update the track. The track remains floating.
Video: http://www.youtube.com/watch?feature=player_detailpage&v=eThLOb0MMn8#t=941s
Environment
Windows 7 64 bit, version 6.1 service pack 1
Java recommended version 7 update 9
Can confirm.