The game crashes when pushing / pulling a track with a piston while there is rain in the surrounding area.
Reproduction:
1. Place a piston (whether or not sticky doesn't matter)
2. Place a track infront of the piston
3. Make sure the surrounding area is NOT protected from the rain
4. Trigger the pistol with a redstone signal

To prevent this crash from happening, the area around the piston and the rail has to be protected from rain.

This sometimes even happens without rain, I don't know what exactly causes it to happen, but it seems to happen only when I look at the piston while it is retracting. In the rain however, it does not matter if I look at it or not. FIXED AS OF 14w29a/b

I myself don't have any idea why the rain is causing that, but I attached the crash reports for more information & data.

The description is "Ticking Particle" in the Crash Reports of the tests with rain on, but on these without rain its just "Unexpected error" even though the code is always the same:

java.lang.IllegalArgumentException: Cannot get property bco{name=shape, clazz=class arw, values=[north_south, east_west, ascending_east, ascending_west, ascending_north, ascending_south, south_east, south_west, north_west, north_east]} as it does not exist in bby{block=minecraft:piston_extension, properties=[facing, type]}

Which, to me, seems like the game is trying to get the block properties of the rail while there is a piston head in that place.

UPDATE It seems like the game ALWAYS crashes if you look at a rail being retreated by a sticky piston, this happens for clients only, if somebody logged in on a server does that, not the server crashes, but his game.

2nd UPDATE in 14w29b (maybe 14w29a, too) the rain crash still exists while the "looking-at-the-track-while-the-piston-retracts"-Bug is fixed.