When you destroy the bottom of a door while a Zombie is already destroying the top, it create a transparent bloc with the "in-destroying" bloc texture.
See the screenshot to see what it is.

• If you destroy the resulting bloc, the sound will play for 1 or 2 minute. (Don't know why)
• The Zombie is wierd because of my texture pack that I was testing...
• In the screenshot I was in creative but I think it work in survival too.
• I was in Single Player.
• I was in a flat world.
• I was in easy.
• If you kill the zombie after destroying the door, the bloc will stay for about 30 seconds. (See 2nd screenshot)
• If a player or the zombie pass through the bloc, it will disapear.
• The sound seems not to play everytime.

(I'm french)