I was checking new snapshots when I read about 13w09b which got an /effect command. I always test snapshots on my small server. I decided to try it and it worked. And then I decided to make command block with /effect command in it. I placed it, wrote /effect 4ty 5 60 1 (so I would have strength II for 1 minute), but it didn't worked. Then I replaced 4ty with @p (so closest player would have strength II for 1 minute) and it didn't worked again. And then I decided to do the same in singleplayer and it worked.