The evening flight from Malaga to Bristol did manage to carry 110 passengers, as well as all their baggage. But as the remaining EasyJet customers queued to board flight EZY6056 at the Costa del Sol airport, they were forced to watch the aircraft set off down the runway.
The astonished passengers, who were left in limbo between the departure gate and the plane, told how they had to bang on locked airport doors for minutes to alert staff to their plight.
EasyJet said the problem occurred on the 9.15pm flight last Saturday due to "an issue at the boarding gate" and the airline was still trying to work out how airport and airline staff contrived to lose a quarter of the passengers without noticing.
The passengers' luggage and boarding passes had all been scanned at the gate before the missing 29 proceeded down the walkway. According to one report, the group had reached a stairwell when a member of the ground crew drew a security ribbon across their path to the plane. Assuming it was a temporary wait while passengers who had already boarded were seated, the group queued patiently - until they looked outside and realised the plane had gone.
One passenger, Iain Locke, 58, of Pensford in Somerset, said the group were waiting in the corridor for 20 minutes. He told the Bristol Evening Post: "I turned round to the guy beside me and said 'I don't want to alarm you, but the plane has gone'. I just couldn't believe it.
"I've flown a lot and I know there are measures in place for the pilot and crew to check the correct number of passengers are on the aircraft. I find it unbelievable that a flight could take off with passengers missing."
He added that being locked in the corridor was "a bit freaky" when their abandonment became clear.
A spokeswoman for EasyJet said 19 customers took the next available flight to Gatwick at 10.30pm. The remaining 10 passengers spent the night in Malaga before flying to Bristol the next day.
In a statement, the airline said: "EasyJet has contacted and apologised to the passengers who did not board their flight from Malaga to Bristol on Saturday 5 October due to an issue at the boarding gate.
"We have launched an investigation to understand what happened and have offered flight vouchers and a refund of onward travel arrangements from Gatwick as a gesture of goodwill to the affected passengers. EasyJet arranged for 19 passengers to travel on the next available service to Gatwick and arranged accommodation for 10 passengers who travelled to Bristol the next day."
The luggage was reunited with the passengers after their flights home, EasyJet said, adding that there was no requirement for cabin crew to do a headcount of passengers on the plane.
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