Falling Anvils glitch

I expected that the falling anvil, which was on a tower and there was a block above the anvil block before it fell, fell down normally

But it glitched into the block above and fell down after that (Video: http://youtu.be/DG0ZVXoCbgQ )

1. Get Minecraft 1.5
2. Find a tree (easiest way)
3. place an anvil on the highest log which is not covered in leaves (below the leaves)
4. The anvil (as a falling entity) goes up by a half-block and falls down