Extremely annoying, especially since it routinely happens during strip mining — I have to go up and check the strip right above the one I just dug out. What's ridiculous is when it happens on the first layer of the mine, because then the blocks end up on the surface. Nope, blocks teleporting thirty blocks up are not funny. At all. Well, at least they don't just annihilate.
Any specific details? Because otherwise it's "Can't Reproduce/Won't Fix".