Eurotunnel is investigating after passengers were taken from Folkestone to France and back again without being able to get off the train.
Seven vehicles were left on the train on Saturday because staff forgot to unload one of the wagons.
The error occurred after a fuel spill meant one of the wagons was left empty.
When the train reached Calais, staff unloaded the front cars but did not realise there were more cars behind the empty wagon.
Train crews realised what had happened while they were on the way back to Kent and made an announcement but were not able to do anything until they got back to Folkestone.
The passengers, who had to wait a further two-and-a-half hours to reach their destination, have been given a refund and offered a ticket for the next time they travel.
A spokesman for Eurotunnel said: "We have apologised profusely.
"Something like this has never happened before in our 16-year history and it's very embarrassing for us."