A US-bound plane was unable to take off from Heathrow Airport after a mouse was spotted on board.
Passengers on the 10:40 GMT British Airways flight to San Francisco were strapped in and waiting to leave when the "rather unusual occurrence" was announced.
The crew told passengers planes cannot take off with mice on board, and that another flight would be scheduled.
It eventually left about four hours later.
One passenger, Carly, said there was "general disbelief" among fellow travellers when the news was broken.
"I'm not sure people knew how to react," she said.
'Sell the movie'
She said they had been told another plane had been found, but the passengers would have to wait several hours for a gate to be found before it could take off.
Some of the flight's passengers tweeted their reactions.
He also wrote: "I'm going to sell this to the movies. It can be the slightly more pedestrian prequel to snakes on a plane. Maybe this is what lured them on?"
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."
Heathrow Airport said it could not comment on individual flights.
In 2015, boxer David Haye's flight from Birmingham was also cancelled because a mouse was seen on the plane.