While breaking a block with hand, if you throw/pick-up and item the block damage will reset and continue from 0.
Linked issues
is duplicated by 3
Comments

I think he means that the block damage animation will start over
Ah, yeah, that makes a lot more sense. Description was confusing. I was able to replicate that behavior. I also noticed that when switching inventory slots the breaking animation would also reset, and this bug may be related to the fact you are essentially "switching" the contents of your slot. Definitely confirmed, but description could be better.

Martin, changing slots is not a bug, this is clearly intended, see the discussion in [MC-2519]
Ah. Perhaps that behavior is what is causing this bug, then? Since you are essentially 'swapping' the contents of your inventory. Does this make the bug invalid?
Duplicate of MC-2519
Rephrased: If held item quantity or damage updates while the player is breaking a block (example: punching dirt with dirt, picks up dirt in the middle), the block breaking animation resets to 0%.
This is related to, but not a duplicate of MC-2519. It occurs in 1.4.5
Confirmed for 1.7.5, 14w08a and 14w10c
Unable to replicate on both 1.4.2 and 1.4.4. Tested with blue wool, did not reset to white wool.