Sometimes, when not looking directly at mobs or slime blocks, the normals of their faces seem to be inverted. It automatically fixes itself when you look directly at the mob/slimeblock, but it inverses again when looking away.
An example is the creeper on the left in the screenshot.
Is this still an issue in the current Minecraft Snapshot 15w45b or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.