Factual error: Near the beginning of the film, there is a reference to Capt. Merriwether Lewis being summoned to the "White House." That scene takes place in 1804. In reality, the Executive Mansion was not called the White House until 1811.
Factual error: The system devised to get the height right was, in the film, said to have been thought of by the 617 Sqn crews following a visit to the theater. In reality it was devised by the 'boffins' at Farnborough.
Suggested correction: See many previously posted 'mistakes' of this type and the standard correction: this is not a documentary and never pretended to be. It is a war drama and many facts were changed to fit the action.
Those correction are often made in fallacy. Dramas based on historical accounts have liberty to change small or inconsequential things, like adding a person that may have never existed, or change a relationship for dramatic purposes. Despite not being a documentary, unless it's a fantasy film, changes in historical facts are mistakes (for example, a drama can't have the Eiffel Tower in London just because it's not a documentary).
Suggested correction: This was the version given in Brickhill's book. The real story wasn't published till many years after the film was made.