Up-stairs (and I presume down-stairs as well) can spawn as part of the self-destructing wall in the "An Ambush! You're Doomed!" room. After the wall breaks, the stairs are left free-standing, where they don't interfere with enemy movement. They're still functional, however, giving the player an escape route not two tiles from where he's standing when the wall-breaking script fires (or one tile away if he used these stairs to access the floor in the first place). My gut says that this behavior -- in particular, the stairs spawning in this location -- is not business-as-intended.
I love that room! By the time any hero find that room, they're already a badass, so they just laugh and kill the ambushers! Interesting bug though.
Pretty certain this was a known possibility. The room in question uses just normal walls and then casts the digging ray spell to knock them out. Anybody with maxed golemancy can do the same. When stairs are placed, I'm fairly sure the game just checks for any spot of a 2-high wall attached to at least one spot of open floor. Technically, I bet stairs could appear in most of the hidden rooms, too, and it's just not happening very often because they're tiny. The typical floor has probably thousands of wall spaces, but only maybe a dozen that are in hidden rooms. Hmm... I just might need to code a huge hidden room into Interior Dredmorating now to test this theory.
