Mixed vaults.
We see them every now and then in the early game if not in SMC then in pc6.
I got an unusually bad situation in SMC with vault, hearing a classic wind noise I fear the worst, scout the left side but finding no downstairs.
So how bad can it be?
I poke the east side and hear a tension, spot some various monsters to the south indicating a mixed vault, ok often I can sneak or wiggle past the worst stuff and doors near to the north east are the only option barring going back up again.
Lo and behold, a mixed tension room next to the mixed vault, RIP!
Edit: this was of course in ud below SMC.