After making at least one piece, select the core. Click the space next to the "chain" symbol, and put in the number of one of the pieces. Try playing the level, and the game freezes up.
Ya, the core should never have a parent part set, as is it gets into an infinite loop that looks like this: core -> part (core's child) -> core (part's child) -> part....repeat. I'll throw something in there to not allow that to happen.