When making a wither you create the standard T shape and then place the heads. However I noticed a bug while experimenting with the new ways to summon withers such as sideways or upside down. If there are blocks under the arms of the T's, the wither won't spawn. It doesn't matter what kind of block as far as I can tell. Before this version the wither didn't care what block was under the arms, as long as there was the T and the heads. This effects all forms of the wither spawning as far as I can tell. Sideways wither's won't spawn if there are blocks in the corner form if there are blocks in the corner between the tail and the outer heads, and neither will upside down withers if you place blocks above the arms.