If you give yourself or someone else the 'Mining Fatigue' potion effect with an amplifier of over 127, destroying blocks does not work properly.
What I expected to happen was...:
The potion effect would either slow down my mining by a lot or speed it up by a lot (as sometimes values over 127 represent negative numbers).
What actually happened was...:
I seemed to destroy blocks with extreme speed but, if I walked over the places where the blocks I destroyed were, I would hover above the hole as if the block was still there. If I then proceeded to right-click an adjacent block to the hole the block would be restored.
Steps to reproduce:
1. Enter Survival mode.
2. Press T then type "/effect @p 4 30 128"
3. Press enter.
4. Mine away some blocks.
5. Walking over the blocks will leaving you hovering over the blocks, unable to move.
6. Right-clicking a block adjacent to it (the side of the block you right-click must be touching the gap too) will restore the block.