When using the AMPLIFED world generation setting, sometimes trees get generated with the top layer of leaves at y:256. At this height you are unable to place blocks except water and lava. This means that you can not replace the leaves once you've broken them.
How to reproduce:
Generate a new world in 1.7.2 using "AMPLIFIED" world settings and the seed: -7225359505972818499
Break one of the leaf blocks in the top layer of leaves.
Try and replace it.