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.
Continuity mistake: Wallis is showing his idea to the doctor and draws on the sketch of the dam. Shortly after, he shows the doctor the same sketch again and the middle line we saw Wallis draw has now changed position. (00:06:40 - 00:07:50)
Factual error: One of the shots of individual British Lancaster bombers approaching one of the dams is actually and incorrectly an American B17, seen for only a second or two. [At no time in the film is a B17 visible in the British release of the film. However the B17 crashing appears in a version of the film released in the U.S.A.]
Factual error: When belts of ammunition are being loaded into the rear of a Lancaster before the mission are.50 cal. Browning. The Browning machine guns on Operation Chastise Lancasters were.303 calibre. Later version Lancasters such as those used for the film were fitted with the larger calibre Brownings.
