Trying to break block in spawn protection logs warning "Mismatch in destroy block pos"

The bug

When a non-op player tries to break blocks in the spawn protection of a server, the following warning is logged:

[23:55:13 WARN]: Mismatch in destroy block pos: fk{x=-9, y=0, z=-6} fk{x=1, y=2, z=1}

How to reproduce

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

   /op Notch
   

3. Join the server and try to break blocks within the spawn protection
4. Look at the server log
   The warning was logged