A flight from Seattle to Dallas was forced to turn back after a human heart was left on board.
The Southwest Airlines plane was flying over eastern Idaho and 600 miles into its journey when staff discovered the "life critical cargo shipment".
The organ was meant to have been delivered to a Seattle hospital after being transported from California.
Airline spokesman Dan Landson said: "Once we realised the error we immediately worked to return to Seattle."
The Seattle Times reported that the flight's captain told horrified passengers the cargo was a human heart.
The organ can typically be stored for between four and six hours for a transplant.
A doctor who was among people on board the plane – but not involved in the shipment of the heart – described the shock of fellow passengers.
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He said people checked their smartphones to find out the length of time a heart can be stored before it is no longer able to be used in a transplant operation.
He told the newspaper that the incident was a "horrific story of gross negligence".