When the Geese take over the prison command post; the Cuban officer removes the cigar from his mouth when the camera changes angle he removes it again.
During Fynn and Coetzee's first meeting in the pub, the volume of beer in their glasses increases and decreases between shots.
Shawn's shadow is almost vertical when he's standing next to the airplane towards the end. This means it's about noon, but he says "We're losing light, I'd hate to try to get this wreck off the ground in the dark."
When the mission plan is outlined by Richard Harris' character, the Geese are to infiltrate the Zembala prison in a "pre-dawn" raid allowing them to gas the main garrison of 200 men whilst they sleep. All of the scenes of the prison attack are, however, clearly shot in broad daylight, with harsh shadows and clear visibility, yet it is still meant to be night because most of the garrison are asleep in bed.
When Richard Burton (Col. Faulkner) is first discussing the job and drinking large slugs of Sir Edward's whiskey he comments "I work dry" yet when in the preparation phase in Swaziland he and the other officers enjoy a beer at the end of the day.
Hardy Kruger's character says that his crossbow will send a bolt right through a man at a hundred and twenty yards, yet when he shoots the sentries from a much closer range, the bolts only penetrate a few inches. Also, if the crossbow was that powerful, he wouldn't be able to cock it by hand, which he does easily.
During the Medical Orderly's final scene he runs out of ammo for his Uzi.
The weapon, however, doesn't click once but three times which is impossible with a real weapon.
All of the three clicks were added post production to enhance the fact to the audience that Witty ran out of ammo.
During the final battle, hundreds of shooters fire thousands of rounds at each other, yet not a single person is seen reloading.
Several Simbas are seen being catapulted high into the air by hand grenade explosions. No standard issue grenade has the power to do this.
It is assumed by many viewers that the Mercenaries were using L1A1 SLR's, when in fact they were using FN FALs on which the L1A1 is based. The FN FAL is selective fire thus can fire on full-automatic where the L1A1 is semi-automatic only.
It is obvious that Richard Burton is wearing a toupee throughout the film.
The black London Taxi (of which there are many thousands) that Faulkner hails in the street to visit Janders house has the registration (licence) plate JUL342K. Some hours later when Faulkner and Janders are in a taxi together when tracking down Fynn, it just happens to be the very same taxi, which they also would've hailed rather than booked. (Clearly, all the exterior taxi shots were filmed at the same time with the same vehicle)
The hammer on R.S.M. Sandy Young's Colt .45 is still down after he fires a round into the dirt to get the exhausted soldier moving again during the training sequence.
When Shaun (Roger Moore) confronts the two mafia men, the bodyguard goes for his pistol and is shot in the head. The sound Shaun's pistol makes sounds like a silenced shot, but his pistol has no silencer on it.
Military rank and unit insignia would not be worn by a mercenary unit, nor would its members likely wear the same uniforms. Even if they were an actual military unit, its members would remove any rank and unit insignia prior to combat operations for security reasons.
During Witty's big action scene he cries out: "Come on you beauties, where are you?" Although that is the spoken line, the footage shows him, (partially concealed behind a bush), actually firing his weapon. The weapon sounds were removed from the brief footage and the dialogue inserted, but the gun-smoke and weapon shaking from the firing is visible.
During the meeting between Faulkner (Richard Burton) and Matheson (Stewart Granger), there are two audio anomalies. Firstly, when Matheson says 'It's good of you to come at such short notice', his lips clearly don't move. Also, when Faulkner uses the phrase 'rushed meal on the plane' his lip movements don't fully match with these words and his voice tone changes, indicating this line of dialogue was re-recorded by Burton in post-production.
Flynn confronts the mobster over the bad drugs that Flynn has been inadvertently delivering for him, and tells him, "Now down on your knees and eat!" to force him to consume the deadly drugs.
As the mobster does this he can be seen saying something, but nothing can be heard.
After the plane bombs the bridge, splitting up the men to opposite sides of the river, they shout across to each other to make their way alone and meet up later after Burton's group crosses somewhere down river. But the river bed is dry (as Richard Harris explained earlier) - they could easily have walked across right there.
Faulkner is a colonel but he wears the insignia of a captain.
When the money is in the briefcase near the end of the film, and is on the coffee table open, Colonel Faulkner puts his hand in it to finger it, but Sir Edward could easily have slammed the lid of the case down on his hand - an experienced mercenary would not allow such a possibility.