Shifting passages is known for its constant wall production. In my zealous victory fervor, I like to occasionally run around and clear out as many remaining monsters as I can.
In this case, by killing monsters post-boss, the walls spawned on top of the end boss trophy and exit. I could walk out the main entrance with a "win" scenario, but I could not retrieve the end cash bonus because of this. I don't know if walls should stop forming after both bosses have been cleared, or if a wall simply shouldn't be allowed to spawn on top of the exit and trophy. You can see my sadness below:
Then again, it may just be a cautionary tale to the overzealous victor.