I understand this was a bug in 1.4 and it was reported but it is still present.
When you dig a block with your hand/block or item, if you switch to a different block or item with a different id it resets the breaking of the block.
Please Fix.
MC-26637
Breaking Resets When Switching Blocks/Items
Resolved
Duplicate
0
jim f
(Unassigned)
Unconfirmed
Minecraft 1.6.2
Retrieved
Duplicate of MC-3449, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being resolved as duplicate.