How it happened:

I was fighting spiders, occassionally blocking them with my shield. Then suddenly when I wasn't blocking, the spider was knocked back without me taking damage, even though I wasn't blocking. It jumped at me many times without me taking damage with no sound effects. The spider wasn't able to kill me. Then I started blocking (right clicked once) and mobs were able to damage me once again.

It has happened me twice now in this snapshot.

I don't know how to produce this bug, but it does exist.

UPDATE: Haven't played for a while. Tested in latest snapshot and managed to capture footage.
I can even sprint and attack when this happens and I'm still immune to all damage, however it affects my shield's durability.