An uncompleted fifth-floor quest that Justice Light believes stripped the changelings' copying restrictions.
On the fifth floor there was a quest involving a Quetzalcoatlus ghost. Had the Changelings survived an encounter with the ghost, the system would have allowed them to add the demi-god to their Changeling Library. The whole setup never came to pass.
Justice Light's working theory is that the gating was removed anyway, in anticipation, and never restored. As a result every changeling sent down to The Ninth Floor can now mimic just about anything they manage to touch — including procedurally generated creatures like the Sluggalos, which had never been permitted before. Combined with their retained memories of past iterations, Justice calls it terrifying, and it is the specific observation that convinced him other locked doors in the game must be open too.
The abandoned quest whose leftover permissions broke the changelings' copying limits.