Creepers don't take melee damage and don't deal explosion damage

What I expected to happen was...:
When I attacked a creeper with an iron sword, I expected to kill it or (if I was too slow) that he exploded and dealt damage to me.

What actually happened was...:
I couldn't hit the creeper, like it had no hitbox, though I was close enough to bump into it and push it. It then exploded, but the explosion only destroyed the blocks around, without dealing damage to me.

I don't know what triggers that bug, it isn't systematical. However, it happened several times in different game sessions.