Breaking Blocks
Press and hold mouse1 (dig) next to a block, then move your crosshair onto the block in order to start breaking it. While still holding mouse1 move your crosshair away from the block. The block stays in a broken stage (at least clientside). If you move your crosshair onto the block and start digging, the break-stage of the block will reset. However, if you repeat the initial steps, you will continue breaking the block from where you left off.

I've tried to demonstrate this in a video. For the first block I've added a "mouse1" text to the screen to clearify when I'm holding the left mouse button.

https://www.youtube.com/watch?v=Zp2c5TFu1rw

As you can see I 'punch' next to the block and keep holding the mouse1 button. I don't let go of the mouse1 button until I take my crosshair off the block again.

This bug was introduced with the 1.8 version and is still present in the lastest version. As you can see, the bug does not occur in the following video which was recorded using the 1.7.10 version.

https://www.youtube.com/watch?v=w3ufZ5lmBEY

I take the same exact steps, but the break-stage of the block resets as soon as I move my crosshair off the block.

Swing/Break Delay
If you compare the videos closely you may also notice that when I move my crosshair onto the block on v1.12.2, there is a slight delay before the swing animation and action of breaking the block happens. This is not the case on v1.7.10.

I find it hard to believe that this was done intentionally in the 1.8 update, since it only makes mining feel less smooth. Many casual players may not even notice it, but if you ask some of those stubborn players who refuse to update to the latest version why they're still playing on 1.7.10, this is one of the reasons they would give you. (Although many probably wouldn't be able to speficially identify this exact bug). Anyway, that's a whole different discussion.

I hope I was able to provide you with sufficient information to look into these bugs. If you need anything else from me, feel free to ask.

Thanks,

HalfCreeper

EDIT: The reason I'm reporting two bugs in one report is because it seems as if they could be somehow related.