Where the changelings likely lost their copying restrictions and where Louis leaked the naga secret.
The fifth floor comes up twice in this chapter, both times as the origin of a present problem. It hosted the quest involving the Quetzalcoatlus ghost — a setup that never came to pass, but which Justice Light believes stripped the Changelings' gating and left them able to mimic almost anything they touch.
It is also where Louis told Juice Box what Carl and Donut need to do to defeat the naga, and why it matters so much. That leak gave Team Retribution the strategic knowledge to keep its options open through every subsequent floor. Two unrelated fifth-floor events, and between them they explain most of what Justice is now negotiating around.
The floor whose leftovers — a broken quest permission and a leaked secret — shape the ninth floor's politics.
Also appears in books you've already read — spoiler-safe to their endings: