Me and a guy were playing hardcore mode together, and were attacked by Wither Skeletons. I turned around and ran, then when I came to a dead end, turned and pulled up the shield to block the Wither Skeletons attack. I was holding it up for a whole second, yet when it attacked, it went right through my shield, hurting me and causing me to die. I was very angry at this, as I KNEW you could block their attacks, as I had done it just like 1 minute prior.
It was hardcore, so I could not respawn, I, however, decided to reload a backup because I died to a glitch. I normally do not allow reloading backups in hardcore, but I felt as if I deserved it since I was killed by a bug.
After reloading, the exact same thing killed me again, a Wither Skeleton's attack going through my shield. I was even more angry at this point.
After this I literally went in creative and summoned a Wither Skeleton, gave myself a shield, and tried to block it's attacks. Every single attack was blocked, proving that their attacks can be blocked.
This bug caused me to be killed twice in hardcore mode.