Look at screenshot.
What I did: put a minecart on one of these two rails
What should happen: minecart should fall
What happens: it loses momentum just before reaching the end of the track and sits on top of the block. Sometimes it falls a little bit lower and gets stuck in a block.
Also, if I put 2nd minecart, it pushes the first and first falls on the ground and gets stuck in a block.
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Can confrim with my screen shot (2013-09-12_12.35.33.png). this has been the case for 1.6 as well (not sure if it goes back farther). At the same time, I utilize this bug for my automatic minecart station (pictured in screenshot).