Mobs are disappearing (moastly Irongolems) and then you cant build where they stood.

Mobs are disappearing mostly Irongolems, and when they have disapearaed you can't build where they stood. You know that thay are ther because sometimes you can blow them up with TNT. I think they disappeared in a lag spicke. My word got destroyd because of this bug, becsuse I couldn't build where I wanted.