When mining a block, any block, if a FallingSand entity blocks your view but then continues falling past the block, and you continue mining the same block, it will leave your 2nd progress on the block until you mine it again or mine another block.
And this did occur commonly when I was trying to clear out a stone room underground, right under my... sugar cane farm...
GIF is included below:
https://www.dropbox.com/s/gxy079blwvaa19n/newglitchy.gif?dl=0
(sorry you have to download it to see it)
Is this still an issue in the current Minecraft Snapshot 15w45a or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.