Haven't played for a while, but I remember that when dupes carrying something over a long stretch of stairs, it gets more and more likely that they will loose it...catching it mid air...loosing it again....fetching it midair...stuck in an endless loop until the bell tolls for bedtime. Then they just let whatever they were carrying fall down and go to sleep.
The (approx.) length where I encountered that, was when they had to climb the (~) 10th stair.