Attempting to place a vanilla slab on snow results in:

The game crashed whilst unexpected error
Error: java.lang.IllegalArgumentException: Cannot get property PropertyInteger{name=layers, clazz=class java.lang.Integer, values=[1, 2, 3, 4, 5, 6, 7, 8]} as it does not exist in BlockStateContainer{block=minecraft:grass, properties=[snowy]}

forge 1.12