Placing blocks in prefab mode behaves oddly for me (OS X world builder): When I place a block, nothing happens. When I place another block, I am snapped back to where I was when I placed the previous block, and that block shows up. Rinse and repeat - always one step ahead of the WB server
Does not occur when running the WB on windows. Does occur with the windows client when I tunnel to my mac's WB
Edit: now it's fine on my mac.