I have a debug flash player, so, that's how I caught the error .

RangeError: Error #2006: The supplied index is out of bounds.
at flash.display:isplayObjectContainer…
..
at primitives::LookPrims/primGoBack()
at intepreter::Interpreter/evalCmd()
…
…
at Scratch/step()

Seems to be a previous costume block. I suppose when you called either getChildAt or addChildAt the player miscomputed the index, based on the message.