My mate is a BA pilot ... No really he is.... He once had the heating system for the wings fail. Basically it means that the wings freeze, and get too heavy, and bring the plane down. After half an hour of failure, they were rehearsing the emergency landing procedure, and had identified land to make the emergency landing on. They were five minutes off starting the landing when they got the heating back on. A couple of weeks later, the same thing happened to the pilot who ended up crash landing on the runway at Heathrow.
Bloody hell, I've got to take my hat off to these pilots, the skill that they show when landing in such adversity is amazing