In the map (attached), while I was riding in the minecart and travel forward, the whole world will stop when I reached the third command block.

WHAT I EXPECTED TO HAPPEN WAS:
the minecart activate the detector tail and the command block works regularly as the first two.

WHAT ACTUALLY HAPPENS WAS:
the minecart stopped on the third detector rail, and the whole world stopped. The only way to get out of this is to quit to the main screen and open the map again. While you are sitting on the minecart, the game stops, if the minecart runs by itself, it's fine.

STEPS TO REPRODUCE:
I'm not too sure about this..
But the easiest way is to open the attachment, unzip it, and drag it into the .minecraft/saves folder. then open the map in 15w49b.
This also happened in 15w47c.