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.
American versions had "Trigger" dubbed for "Nigger" (the name of Gibson's dog, and one of the code words). The Morse code received in the Ops room was still "Nigger" though.
In US prints (see trivia entry), a Flying Fortress appears at least once in place of a Lancaster.
During training flights shown early in the film when crews knew nothing about the target or weapon, Lancasters are shown carrying the bouncing bomb that was used on the actual mission.
When Barnes Wallis starts to pull the blackout across it is clearly daytime outside. He stops and walks through the French windows and suddenly it's night time and dark.
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.
On the run-in by the bombers, enemy anti-aircraft defense appears to be cannon fire, however, the Germans used flak, which took down several of the bombers before and after the bombing mission. Other bombers were lost through ground contact or upon hitting transmission lines. There were instances where the flak guns were depressed below the horizon to reach the low-flying bombers.
Before the raid, the Lancasters are shown being loaded with machine gun ammunition. The bullets being loaded are 50 calibre, while the Lancasters, at that time, only used .303 calibre machine guns.
There was no tidal wave in Kassel as shown in the film. The water was only 20 cm high in the old town. The industrial areas were not reached. The only thing that was destroyed was farming villages. Most of the victims were Allied prisoners of war.
Before the first successful bomb test, Gibson tells Wallis that there is a new moon and he should make a wish. He isn't referring to a calendar new moon when the moon is invisible, but rather the first appearance of a thin crescent, the sight of which traditionally prompts a wish. But as Gibson says it, they look in the direction away from the sun, which is behind them. A "new moon" of this kind is always close to the sun, visible low in the sky after sunset. So in any case it shouldn't be visible with the amount of daylight seen.
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 and a strict disciplinarian with little personality.
However, changing a personality trait on a character in a fictionalized account of real events is artistic license and cannot be considered a goof.
When in the upper briefing room where Gibson is addressing his crew, on the right hand wall is a painting that depicts the actual briefing taking place.
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.
Guy Gibson is smoking a pipe in a first class railway compartment. The pipe smoke goes out the window toward the engine, i.e. in the opposite direction of what it should.
The "full size tests" show a de Havilland Mosquito dropping a Highball prototype from a height very much greater than the 150 feet mentioned in the script.
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.
When the plane with the markings AJB catches fire (presumably from anti-aircraft fire) shortly after crossing the coast, the plane bounces before exploding when in reality it would explode when it hitting the ground (1h 32m).
When the first 3 aircraft are taking off from RAF Scampton, they don't have any bombs on-board, yet when the camera changes to film the undercarriage the bombs are visible.
When Barnes Wallis meets Summers and they decide to approach Bomber Command, parked 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.
When testing the use of spotlights to determine the aircraft altitude, 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.
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 the first dam is breached, during the fly over back over the reservoir, underwater jets can be seen in the miniature, driving the flow of water through the gap in the model dam.
The Castle above the Eder-Dam is shown on the southern-side of the lake. In reality, this castle is on the northern 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.
The train showing being derailed by the flood is running on the left-hand track. British trains do run on the left, but German ones run on the right.
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.
A microphone dips into the top right corner and pulls back up at 45:24, just as Wallis says he thought inventing the bomb was a terrific idea "at one time." This is quite visible on the Blu-Ray version.
When Gibson meets with Wallis at the final drop test, he tells Wallis that they have been able to fly at 60 feet. Wallis is astonished, though of course very pleased.
However that was one of the major requirements, in order to prevent the bomb casing bursting as it did when dropped from higher up.
Surely Wallis would have been told in advance that "Mutt" was able to fly at 60 feet for the drop test - especially as Gibson told Wallis that they had been doing it for a week - otherwise there wouldn't have been any point in conducting the test.
This is really an error by the sound dubbing crew.
It's been mentioned that in the US version Gibson's dog is now referred to as 'Trigger", and Gibson sometimes actually did this himself.
The same thing has been done In the version now shown in the UK.
However when Gibson meets up with Trigger after completing a flight, he calls him "Trigger' twice, but then mistakenly refers to him by the 'N' version of the name. It appears this was overlooked when the dialogue was re-dubbed.
When the doctor reads the mercury thermometer at the beginning, he is holding it over a lamp. The incandescent bulb would certainly provide light, but also enough heat to make the reading useless.