When Barnes Wallis meets Summers and they decide to approach Bomber Command, outside the testing chamber is a Fiat 'Topolino' (the original version of the Fiat 500). This is very unlikely to have been imported to Britain before the War and certainly it would have been impossible during it, Italy being an enemy nation. Possibly it was chosen as an authentically 1930s style car that was readily obtainable in 1955.
After the first failure of the full size test bomb, Guy Gibson, Bob Hay and Barnes Wallis are saying goodbye to each other. Bob Hay's mouth moves but what he says to Barnes Wallis is not heard. (at around 25 mins)
When testing the use of spotlights to determine the aircraft height, a crew member is giving instructions to the pilot to carefully fly lower, but in the middle of the sequence the aircraft is seen to turn sharply to the left, which would mean the spotlights would no longer be pointing downwards and giving instructions at this point would be useless.
Guy Gibson was depicted as congenial, friendly and gregarious. However people he worked with, both air crews and ground staff considered him to be a loner, a strict disciplinarian and having little personality.
Gibson and Whitworth are sent off to select pilots for the new squadron. Their first selection is Les Knight, we are shown a photo of the actor Vincent Ball but when we meet the character Les Knight he is actually played by Denys Graham.
Prior to the Raid, Henry Maudslay brushes his hair and puts the two brushes together by their bristles. After the Raid, from which Maudslay does not return, a shot of his room shows the two brushes side by side.
Cochrane tells Gibson to try out the new bomb-sight "on the towers of the Derwentwater dam". He means the dam of the Derwent Reservoir in Derbyshire, about 150 miles from Derwentwater, which is in the Lake District and, being a natural body of water, lacks a dam.
When the planes attack the Eder dam, they fly over Waldeck Castle towards the the dam. In reality, by overflying the castle, directly towards the dam, you would fly almost directly along the length of the dam. You would have to bank almost 75 degrees to port to approach the wall the way its shown in the film.
Shortly after the Lancasters take off from RAF Scampton (which is north of Lincoln) for the raid, they fly over Lincoln Cathedral. However they're shown flying over the Cathedral heading north (the smaller twin Cathedral towers are at the west end of the building) and back in the direction of Scampton.
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.
In two shots of flights of three Lancasters taking off on the mission, each has a gap in the ventral fuselage where the bomb should be. Later shots of the bombers in flight have the weapon showing as a semicircular bulge in the same place - the silhouette of the bomb was presumably added in post-production, with the take-off sequence forgotten about.
The Lancaster shown flying over the Derwent Reservoir is not a Dam Busters aircraft, in that it has bomb-bay doors and a mid-upper turret, both missing from the AVRO Lancaster Mk.IIIs Type 464 (Provisioning) aircraft used in Operation Chastise.
The spot lamps are positioned at either end of the plane so that the beams would cross at a certain height however during the testing the beams are clearly parallel to each other which means either the lamps are not in a straight line from each other or the aircraft is flying side ways.