A British Airways flight had to be stopped on the runway to ditch a mouse that stowed away.
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With roaming rodents banned from flying under aviation rules, another plane had to be found for the human passengers.
One flyer, named only as Carly, told the BBC the air crew announced "a rather unusual occurrence has occurred".
"I'm not sure people knew how to react," said Carly. "There was general disbelief."
Another, Mark Watt, wrote on Twitter: "Just had my flight to SFO cancelled because of a mouse on board the plane. Could it not get a visa?"
Passengers took off four hours later.
British Airways said: "We know almost everyone wants to fly with us to San Francisco, but on this occasion there was one very small customer who we had to send back to the gate.
"Everyone with two legs is now on their way to California, and we are sorry for the delay."