Ok so I created a new world with the latest snapshot> I was cuttin down some trees and whatnot, then I climbed on top of the trees to get a load of my bearings, but then I noticed tree saplings and some of the leave blocks started decaying (disappearing) weird.
If i remember i think if a leaf block is 4 blocks away from a wood block it starts to decay, so i guess the generator could of made a mistake. On the other hand you might have a mod?
No. no mods
its with the new snapshot so that could be it