I don't know if this bug is new or not. I looked through the SMW bug page, and I think it might be the "block factory" one, but I'll go ahead anyway...
It was in Vanilla Dome 3, right at the halfway point. After the koopa was dead and the red shell was still, I picked up the shell and tossed it upwards at the blockk that contained Yoshi. The shell missed the block, but just barely, and hit just to the right (the red switch hadn't been pressed, so it was just the outline.) And there, to the right of the Yoshi block where the red outline was, appeared another Yoshi block.
That's it. Is that the "block factory" bug?
Go Moon!