When pointing at a particular block from a distance that should be too great to mine it, the game still counts it as "taking damage", so to speak, but the block will only break once you are in range of it. This can result in what looks like you instantaneously destroying a stone block with your fist.

To reproduce the bug:

Locate a block that is easily distinguishable.
Position yourself so that you are too far away to mine it.
Select a non-mining tool or use your fist.
Hold the mouse button down while pointing at the block, for 10 - 15 seconds. Do not let the mouse pointer leave the block.
Walk forward until you get within range of the block. The block should either be destroyed immediately or visibly jump to some percentage of being destroyed.