my builds are moving from one place to another

i'm guessing it's a bug that came with the new "pre 6".i built a wall a few day ago and when i swiched to the new pre8 every thing was the same, but after a good hour of playing i went to the nether to get a pick i left there and when i got back i saw that parts of my wall had been moved to difrent locations (all within a 30 blocks of its original locacion) i'll provide screen shots.
i'm sorry if i missed spelled some words, its because i am from south america