When monsters follow you up or down the stairs and there's not enough room for them on the destination level, they arrive one-by-one when the stairs space is free. However, if you manage to leave the level before all of them arrive (by either swapping a pet onto the stairs or immediately teleporting to another set of stairs), they will show up the next time you arrive on a level via any set of stairs. This doesn't make sense, as the monsters have effectively followed you through a level they weren't even able to reach. Since you can limit the number of available spaces with wands of door creation, this bug can be abused in a few ways: getting hostile monsters to "follow" you through the wilderness, and getting companions to "follow" you from D:47 to D:50.