With the new [data] value of the /give command, you can receive some previously unavailable blocks such as tiled Big Mushroom blocks. However, after placing them, they ignore that data.
This may be the case for a certain range of blocks, and probably items (for example, custom Spawn Eggs don't work)

How to reproduce:
1. Make sure to be able to use commands.
2. Use the command /give @p 100 1 1 to give yourself a special Red Mushroom block.
3. Try to place it, and you'll see that there's no texture anymore like the one the icon shows.

In addition, custom mushroom blocks are stacked together with each other in a single stack even if they had a different data when using the command. The first icon will be shown in this case.

Update: it IS possible to summon custom mushroom blocks using the custom FallingSand data /summon command, but is not really a convenient way to do so.