The Situation:
I had a full set of Diamond Armor, a Shield and Blocking from a spider.

The Result of what I wanted:
I successfully block the spiders' attack, but occasionally take a half heart of damage. (Indicated by the health bar flashing)

The ACTUAL Result:
I did successfully block some of the spiders hits, but the one I didn't block my health just magically went down by half a heart. No white flash, no flesh sound or anything. I was holding down block key the whole time and that sometimes the damage seems to penetrate the shield and when it does, no animation is shown on the health bar.