how about of you are using the Tunnels of Nechrubel expansion then a pit swallows you into the tunnels in this situation?
I am working on another small pnp game where this kind of thing happens - it is a side-scrawler and the floor collapses if you hit a dead end
This does seem to he a very rare occurrence, though - I never had it happen on 10+ pre-repease playthroughs of BASILISK!