More info please?
Yes; I made sure of that. However, it is below a fence and above a pinewood plank, if this also means anything.
I also had a similar problem, but with a few changes:
The Command Block is set at a 1-second interval.
The /spawnpoint command sets ALL online players to an absolute position.
The spawn is in neither solid nor liquid blocks; it is in air blocks.
I never seem to reproduce mine on a different world, but the bug is still on the world I am on.
This reminds me when I updated a Beta 1.7.3 world to Beta 1.8.1; some chunks have chopped mountains due to the new terrain generation code.