Can't believe how fast I rushed through making this, but I meant what Tyler said in MC-2588 in the comments. If the quantity of the item (the one you are using to break the block) changes midway through breaking a block, the progress resets. (eg. the first block breaks, and you dig the block behind it (with same block as the one broken). Then the new (broken block) adds up to your stack, updating the number, and thus the progress repeats. )
Also the progress can reset on servers if the lag is severe enough that the durability of a tool changes after the block broke. (The damage value on the tool is changed, since in the previous report they said that the bug was fixed so that progress resets while changing blocks/ same block but different data value.
when the hell will this be fixed?