Anvil Bug

To try out how much damage a falling anvil does I made a flatland world and
built a construction with a piston and redstone to let anvils fall on me.
I put about 6 anvils on top of each other and let them fall on me.
The picture shows what happened: They made the normal falling damage but
then they didn't damage me then. I could walk out of that stack like normal, though I stood inside of them.