(At least when water is pushing the mob) mobs can be hit through corners.
In the screenshot you can see me hitting a creeper through a corner. The white particle can be seen (of hitting a mob)
I was able to reproduce this in 16w44a with 2 skeletons, although it happens very rarely. So rarely that I didn't manage to get a screenshot, but I'll add a screenshot if I manage to get one.
Environment
Windows 10, newest Java
This other picture is about me doing the same with barrier blocks. Barrier particles and levers are there to proof that there is barrier blocks there.