This happens to me in most buildings with staircases as steep or steeper than those in apt. buildings. I believe it has an increased chance of happening when you have a momentarily bad connection with the server, and the server/client catching up and resyncing player location may play a role in this.
It's the most common gameplay issue I experience; it's the first bug that's frustrated me enough to feel a need to make an account and report it as well. :P