Hit by mob through solid wall

I was poisoned by a cave spider and walled myself into a 1x2 hole to recover. I was in a mine shaft and backed into a corner created by 3 blocks of planks and the solid rock wall. I put a 2 high column of cobble on the other two sides and a cobble block right above my head. About 3 seconds after doing this I was hit from behind by a zombie. (Luckily only dropping to 1/2 a heart and not dying...) I turned and saw a zombie arm between the cobble and wood walls. It retracted after a second and did not reappear. After healing to 10 hearts, I backed into the same corner to see if it would glitch again, but so far it hasn't. The 'roof' was the last block I placed so it wasn't a case of being hit rights as I placed the wall block.