The Southwest Airlines flight bound for Dallas, Texas, had to turn around and head back to Seattle-Tacoma International Airport after the crew realised someone had forgotten to unload the heart.
Yahoo News UK reports that the captain said the heart should have been left in Seattle after an earlier flight from Sacramento and taken to a hospital.
After the u-turn, the flight landed in Seattle and the “life-critical cargo shipment” was unloaded.
Southwest spokesman Dan Landson said the return to Seattle was vital because it was “absolutely necessary to deliver the shipment to its destination in the Seattle area as quickly as possible.”
He said: “Nothing is more important to us than the safety of our customers and the safe delivery of the precious cargo we transport every day.”
Once the plane was back in Seattle, passengers also had to get off because of a mechanical delay.
It took off again after a five-hour delay, Landson reportedly said.