Nether/multiplayer/random blocks generated

1 created server (12w49a) today and i played happily there with my frends. I was testing new fIreworks and having fun. Then i desided to test them in nether. Suddenly server starts lag like a hell. I lookes to nether and i saw many big holes and they were full of random stuff (Command blocks, fence, spawners, cobble, mosscobble and allkinda blocks). I was scared. I almost lost my test world