Factual error: During the war games they have Jefferson pose as a major in order to facilitate the commandeering of the ambulance and the jeep. Regardless of their personal feelings in the matter the ambulance crew and the driver and troops in the jeep would be perfectly aware that no black man would ever be promoted to a position of authority in an otherwise all white command. We may find it repugnant today but the US Army was rigidly segregated during World War 2 - and it stayed that way until 1948. Jefferson may have been inducted into a special unit like the Dirty Dozen but considering that the future of the entire mission is riding on their success at the games, throwing it all away like that makes no sense at all.
Factual error: When M and his entourage arrive at Bond's estate their cars are surrounded by sheep. They then drive on down a unsurfaced road to Bond's house, past a pride of lions he keeps on the grounds. A series of sweeping overhead shots shows us it is all one huge field, with no fences or the like. Those sheep aren't going to last very long .
Other mistake: Bandoleer rounds don't fit any weapon in the show. When Pierce tries the Gatling gun, there is no magazine in it.
Revealing mistake: When the farmer is going to shoot the sign the bank put up, you can very well see the mark where the hole is going to appear.
Continuity mistake: The scene where Col. Hathi inspects his troops, he leans on his stick and it breaks. However, in the next scenes, the stick you see is sometimes broken and sometimes it is still complete.
Plot hole: Towards the end of the film when Newman and the remaining stagecoach people are in the cabin at the top of the hill, Richard Boone's character sends one of his two henchman (the Anglo, not the Mexican) around to the back of the cabin to apparently surprise them from behind. Then Newman's character walks down the hill and Boone and the Mexican henchman are all killed in a shootout. Everyone comes out of the cabin, comes down the hill and the credits roll. I guess that now, almost 40 years later, that poor Anglo-henchman is still behind the cabin, waiting for them to come out.
Visible crew/equipment: As Quatermass drives back to the spaceship in a cab, just before it is to be drilled, the film crew can be easily seen in the cab's windows. A woman with a white cap can be seen holding a script. Later when the police use a megaphone on their car to announce the spaceship is a hoax, you can again see the same woman with a white cap in one of the windows of the police car.
Continuity mistake: There's a scene where Audrey Hepburn's husband Sam is leaving their apartment. Standing on the pavement outside he hails a cab, which pulls up through a (quite sizeable) puddle, splashing him with water. Opposite him on the other side of the road are parked two cars and the villains' Volkswagen van. There's a quick cut-away exchange of "pleasantries" between Sam and the cabby before it drives off. The next shot has the villain (Richard Crenna) emerging from the van, crossing the street, and making for the door to their apartment, only all of a sudden the puddle has disappeared, as have the two cars which were parked behind the Volkswagen van.
Continuity mistake: Jim Dale and Anita Harris think that Barbara Windsor is going to jump off the roof and race across the road into the nurse's block. We see Anita Harris is wearing dark seamed hosiery as she runs up the stairs. A few minutes later she hangs down from the roof and the seams have disappeared from the backs of her legs. (00:59:00 - 01:01:55)
Visible crew/equipment: After Lucifer talks to God and doesn't know what is happening and Saint Peter comes to escort him out they get into a discussion why he failed the entrance examination, you can see a reflection in glass between the camera and Lucifer and a man who does not appear to be St. Peter. (Maybe director Stanley Donen?) (01:39:25)
Continuity mistake: During a session in class when they were discussing marriage etc, Potter and Denham can be seen in the background leaning in their chairs against the wall. The discussion then involves them and the Teacher played by Sidney Poitier. In the scenes that follow it shows them leaning on their desk when the pair of them are talking to the teacher, and when the teacher is talking it shows them in their original posture with their chairs still tilting.
Other mistake: After the "fat fat fatty" scene, Franz shoots his way in, starting with three bullets into the door lock. Over the next scene he fires about twelve shots, despite the fact he's firing an 8-shot Luger.
Continuity mistake: When JB talks to Ms. Jones on the intercom after talking to his wife on the telephone, he asks where the "You know" is. She says "It's in your top right-hand drawer." You can see him moving to his left.
Continuity mistake: Millie's beads have a mind of their own. They do not stay in place even when she is standing still. Besides the funny scene during the title credits (when it is deliberate), and when Miss Dorothy registers, they also move during a scene when Millie displays her thoughts on Miss Dorothy's beads when the two of them go into the elevator for the first time.