Zombie. when I initially started my first play through of terraria I didn't really know how the "Walls" worked. Thought if I put it; no monster would be able to attack me, boy was I wrong I yelled "Haha! take that zombie" as I had put the wall all over the area I was on, confident that it couldn't pass through.
but .... it did so for the next 5 minutes was just me screaming in confusion as to how the zombie got through my wood "walls" in the first place only to find out that you need to put blocks to block the zombie...... yeah