Roofed Oak Tree grwoth can destroy blocks under the saplings

(Woha, no issue about this on the bug tracker yet)
Well, I think the video does explain it the best:
http://www.youtube.com/watch?v=M-gdbqMihAE
Basically: The growth of Roofed Oak trees can cause all blocks to be destroyed, even bedrock and other unbreakable blocks.