Well known to dansdeals readers is uniteds plan b {which I recently used a few times and was upgraded on flights which had been showing completely sold out. Right before cabin doors were closed the groundstaff come to economy to upgrade my whole family to business class !!}.
Another slightly similar phenomenon I recently had was with air canada using aeroplan points . One friend of mine booked ewr mel via vancouver for 67.5k in premium economy. However only the short haul leg had been available in premium. At the gate he was given premium economy for the long haul also. Without any asking. The same thing for a family of 4 paid 80K for business however only the long haul [mel to vancouver] was available. In vancouver at the gate they gave them new boarding passes for business class to newark !!