A "simple flaw" caused by a language difference led to a "serious incident" for a flight from Birmingham last year.
All female passengers whose title was "Miss" were classified as children - not adults - on the Tui flight after a software upgrade, a report said.
That meant that their average weight used for take-off calculations was lower than it should have been.
The difference could have had an impact on take-off thrust, but the report said flight operation was not compromised.
Take-off prep documents told the pilot that his Boeing 737 jet was 1,244kg lighter than it actually was after using 35kg as the average weight of the females involved rather than 69kg.
That "load sheet" is used to determine how much weight is safe, how the plane is balanced, and other important information for a safe flight within safety regulations.
The AAIB report on the incident, external said the reservation system that produces the load sheet had been upgraded in the downtime caused by the coronavirus lockdown last year, when the airline suspended flights "for several months".
Safety officials said the problem was that the software had been programmed in a foreign country where "Miss" is used to refer to children, and "Ms" to adult women.