Insta-mining blocks (e.g. dirt with an efficiency 5 diamond shovel) occasionally leaves behind ghost blocks, of which there still is a server-side hitbox but the client counts it as already dug.
This happens quite often - just going around like this produces one mismatched block every few seconds.
As I cannot reproduce this in any pre-release prior to pre4 (so I can from pre4 and upwards), I believe this is related to the new packet/fix for
See the attached video, reproducing the bug in a freshly created singleplayer world (I also waited for generation to calm down before starting to dig).