I was checking out some of the 14w25b new features like underwater temples and Guardians. I heard that some of the unnecessary data values were removed for some blocks (like inside-out cake, go in 1.7 and /setblock ~ ~ ~ cake 15 and you'll see). I tried to spawn one of the blocks, it just rolled over to data value 0 - as you'd expect.

Then I opened back up 1.7.9 and spawned a few glitched pumpkin stems (pumpkin_stem:15, looks like two paper halves of a pumpkin beneath a stem). I went back to 14w25b...CRASH! Instantly the game crashes when trying to load the world due to a "tesselating block in world"...? I haven't seen this crash error before. Tried again. "Cannot set property aym

{name=age, clazz=class java.lang.Integer, values=[0, 1, 2, 3, 4, 5, 6, 7]}

to 15 on block minecraft:pumpkin_stem, it is not an allowed value." - the game doesn't know how to handle the invalid data value of 15 because it's out of bounds, so it gives up and crashes.