(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.
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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.

Unable to confirm. Please provide the steps you used to reproduce the glitch.

Can't reproduce in 16w36a

Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w43a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.