so, i was playing on my rather modified survival world (just modified with cmd blocks, no mods), when suddenly my shield didn't work, even if i was perfectly aligned.
what i tough it happened: the shield blocked my damage and i would be fine.
what it ACTUALLY appened: the creeper completely ignored the shield and got me down about 3 hearts.

how to re-create the bug: try in survival, try to block some creeper attacks with a shield. sometimes doesn't work