I moved a family into an apartment and for the first few days, everything was fine. Then all of a sudden one of the common staircases sort of stopped working - Sims could go up it, but couldn't go down. They'd stand at the top, freeze for a second, then just turn around. I tried move_objects on to put them all on the pavement outside the building, but they'd just walk straight back into the building and up to the landing they got stuck on. The nanny, paper boy etc eventually leave but neighbours and guests just stay until I can move them when they fall asleep, so I now have three Sims standing outside my building asleep (thanks to move_objects on) - they've been comatose for over a week!

I can't figure out what caused this glitch, since it started a few days after move in, identical staircase layouts in the same apartment complex work fine (so I don't think it's related to the orientation of the spiral staircase or the small 4x2 landing). I've moved the family out because I was fed up of having to use move_objects on every time one of my Sims needed to leave the building, and the build up of trapped guests and neighbours. Is it safe to move them back into the same apartment? I'm hoping the glitch will sort of fix itself as the lot will 'reset' when I move them back in.

That sounds like a stuck "in use" tag blocking the downward path of the spiral staircase. Since the apartment sublot for each unit is created anew every time a new family moves in, it should indeed be fixed if you move another family into the apartment.

However, it's as likely to happen again as not. There may be other methods of removing stuck in use tags than deleting the object with move_objects, but I don't know what they are, and this particular problem is especially annoying since in order to remove the stairs without moving the family out you'd need to use the cheat to enable the relevant build mode items first.

This is why I tend to build apartments with more than one staircase, and possibly an elevator to boot. (Elevators can get stuck, too, though.) In order to add more stairs to your apartment, you'll need to move all families out, enter the lot, change the lot zoning to residential (changelotzoning residential), save, return to neighborhood, enter the lot in build mode again, make your changes, save, change the lot zoning back (changelotzoning apartmentbase), save, and exit.

Regarding stuck visitors (including former apartment neighbors who have been moved out but don't seem to have noticed): Just use move objects to delete them. It doesn't hurt them at all.

I find that sometimes forcing an error> reset on the affected staircase can solve the problem without having to resort to moving out.