Linux, x86_64, Sun Java 1.6
When using an enchanted diamond axe (efficiency iv, unbreaking iii, fortune iii) and hitting wooden block at an angle, block shows damage animation but doesn't break.
1. select tree with 2 wood blocks showing, upper wood blocks hidden by leaves
2. destroy upper visible wood block (leave bottom wood block)
3. destroy leaf block, then repeatedly hit upper wooden block
Wood blocks are destroyed as expected when standing directly beneath.