Mining fatigue potion effect amplifier of over 127 does not work properly

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.