Changing tools while breaking blocks

Excuse me but this perfectly makes sense. Put the first tool in you pocket. Grab an other tool. Start digging. This works as intended. No bug.

No im sorry but when you changed tool while mining in other versions it should continue the process without starting from 0

Possibly you were playing with a modded minecraft ?
The function you've described was never in vanilla minecraft and would give some balancing issues:

• You start mining a block of obsidian with your valuable diamond pickaxe and in the last moment before the block breaks, you switch to your cheap iron pickaxe --> You'd have mined the block of obsidian in a reasonable amount of time and the damage goes completely to the cheap pickaxe.

No, i mean while im mining the block i change the tool in my hand (WITHOUT TO GET MY HAND OFF THE LEFT BUTTON OF MY MOUSE!)

I understand what you mean. You (in front of the computer) keep your finger on the mouse button but your character in the game switches to an other tool. It perfectly makes sense to stop and restart the action. This also avoids an older bug where you can dig a block with a diamond tool and quickly switch to a cheaper tool. This was possible in previous versions. It was considered a bug and got fixed. So no, this is not a bug. The current behavior is intended.

He is actually correct I have had this happen to me plenty of times no matter modded or not. So you my sir (mrheat) are actually wrong and have no clue what you are talking about because this man is correct.

No, mrheat is correct because this was designed to prevent breaking blocks really fast with a diamond tool, etc. and then switching to your hand so the tool does not receive damage. Kumasasa has a good explanation.

This was actually a bug fix. Before, you could do a duplication glitch with dispenser. You used a pick to destroy the block and then change tool while breaking it. When it broke, the dispenser appeared again, but soon it will disappear. When it appeared for the first time you could go into the dispenser and if you did that and spam click an item in a dispenser slot until the dispenser disappears, you get one of the items and you can right click it in the slots in your inventory and fill up it with the item.

This was maybe a bug fix for that mrheat wrote but whatever.

AW damn it, it is a fix, not a bug? Damn.

Jakob, that would be a separate bug with its own fix.

This is happening in 1.5.1 for me. Please update.

Sure it's happening, it would be a bug if it would not happen...