WorldSpawn Bug

I was playing a custom selfmade Minigame with my friend as a weird bug appeared: On the exact coordinates, where the WorldSpawn is, appeared a Oak Wood Block. Also, my friend spawned on the block (when he got killed). He shouldn't, because he has set his Spawn with a bed ~100 blocks away and there are blocks above the WorldSpawn; normally you should spawn on the most top block in the spawnarea, not below.

I have no idea what happened and I am sorry, that I can't reproduce this bug.

PS: Later, I was loocking with the programm "MCEdit" on the map. That's why I can tell that the block appeared on the coordinates of the WorldSpawn.