Mobs with shaders don't render GUI screens.

[sschr15]

Screens such as the inventory or escape screen are not rendered when possessing a mob that has shaders (creeper, enderman, etc.)

This is partially fixed once 0.3.12 releases (shortly), but if there are other mobs from other mods that you find, send a comment on this issue. I probably will look into those mobs to see if I can fix their bugs as well.

[IndieanaJones]

One thing to note about this is that the shaders don't turn off when using third person, which is usually what happens when spectating them. I think having it behave like that would also be a good decision for possessing them as well, since spiders and things of similar caliber can get pretty annoying to pilot with their shader.