*What I expected to happen was...
The wither would not destroy the block
What actually happened was...:
The wither destroyed the block
Steps to Reproduce:
1. Build a full square of barriers large enough for a wither
2. Summon a wither inside the barrier
Linked issues
is duplicated by 6
Attachments
Comments
Confirmed in 07a
Confirmed in 10c
Still happens in 14w32a.
Edit - details: Wither attempted to move outside the barrier cube, and could shoot blue skulls through it (most of those craters are from such). As can be surmised from the screenshot, it was not aiming at any mob or creature in particular. It appears that its proximity block destruction is what destroyed the barriers and allowed it to escape.

Reoccured in 1.8 (MC-73105)

Cannot reproduce in 1.8.2-pre1. Can provide testing world if needed.
EDIT: Just tested this bug in older versions and it occurs in 1.8, but not in 1.8.1 (the blue skulls were destroying the barriers, the Wither doesn't break them when taking damage). Therefore, it should be safe to say that this was fixed in 1.8.1 🙂.
Also the bug fix was added to the wiki page as of December 3:
http://minecraft.gamepedia.com/index.php?title=Wither&diff=next&oldid=760581

Ok, thanks.
Confirmed in both creative and survival