The Unintended Cascade of Events in a Flight

The aircraft had been at a service earlier, which required pulling of the forward and aft laboratory drain mass heater. And it's possible that circuit breakers were inadvertently left open, meaning these heaters weren't operating. The water inside the laboratory drain froze, which caused water backup into the laboratories,. Which then spilled out and then on two critical systems, which then froze  and made it very difficult to fly the airplane.