When I was playing around with hitboxes, I built an iron golem.
I noticed that it's legs were right in a block, and it could walk through it (see first screenshot).
Then. I built a two-block-high wall and hit an iron golem into it. Of course, it went right through (see screenshot 2).
Strange.
Environment
Windows 10.0, Java Version 1.8.0_25
Probably intended. The hitbox of the iron golem and the block overlapped. This is the same if you stand half inside a block, you can still move out of it.