What I expected to happen was...:
The mined blocks to be gone and/or be visible

What actually happened was...:
mined blocks are invisible on occasion (if you are mining really quickly with say an efficiency 5 shovel on dirt) the area where the block was will cause you to vibrate almost when you go through it. 

Steps to Reproduce:
1. Use Efficiency 5 shovel on large amounts of dirt.
2. wait for one of these glitched or bugged areas to appear
3. The block will then be impossible to go through and will not be visible again until you restart the game.