The bug

When an op player hits (=left click once) a block in the spawn protection and is afterwards de-opped, they can still break that block.

How to reproduce

1. Start a server
2. Op any player other than yourself (from the console) to activate spawn protection

   /op Notch
   

3. Op yourself (from the console)

   /op @p
   

4. Set your gamemode to survival

   /gamemode survival @p
   

5. Click a block in spawn protection once
6. De-op yourself

   /deop @p
   

7. Try breaking the block you hit in the previous step
   You can successfully break it