I've observed two other bugs:
One, in the souvenir selection screen, if you click a souvenir (bringing up the confirmation button) but then click Restart, the active souvenirs are cleared as normal but the confirmation button remains and works. As such, once you have access to 3 souvenirs, it is possible to repeat the above process to activate souvenirs as far right as you want, skipping the ones on the left.
Second, somehow defeating one cerberus head and letting it respawn removed the stairs, which were in the same row. Also, while I'm not sure if it's related or indeed a bug, using a mirror card on cerberus heads reflected damage to other monsters. Edit: Refreshing did not bring the stairs back, so I had to abandon the game.