This affects the 1.8 Snapshots later than 30a. May be sooner

The wither will not spawn if there are blocks underneath the arms of the T of soul-sand when trying to spawn the wither in any difficulty.

Representation:
W= Wither Skeleton Skull, S = Soul Sand, X = Empty Space,
W W W
S S S
X S X

If there are any blocks in the X spaces the Wither will fail to spawn.
This was also seen in an Episode of Mind-crack Season 5 from sethbling when they were trying to spawn a wither and it wouldn't work as there was tall grass underneath the soul sand. After they moved the soul sand it worked fine as there was nothing underneath the T

Link to Mind-crack episode:
https://www.youtube.com/watch?v=PZQk9YLoCgA&list=UU8aG3LDTDwNR1UQhSn9uVrw