When breaking blocks, sometimes the mining process resets. This is extremely noticeable in multiplayer (even when your ping is below 100). This essentially makes mining a block twice as long.

It happens when you...

1. Switch tools.
2. The damage value of your tool changes (which happens every time you mine a block)
3. Holding an item and picking up another one of them. (Also annoying, because when you are breaking blocks with that block in your hand, such as dirt, you constantly pick up that block, and thus resetting the mining process every single time you mine a new block)

This bug wasn't around until 1.4.6 and is still in the latest snapshot.

I hope my explanation was sufficient, I know my English isn't the greatest.