While passing the German fort, a hose on the boiler is shot and damaged. Allnut wraps a cloth around the hose and then begins to wrap it with tape. The tape is clearly cotton-based black gaffer's tape, used in movie and stage production to cover and hide set hardware that is not to be seen. Gaffer's tape was invented by Johnson & Johnson in 1942 in response to the military's request for a tape that could be used to seal metal ammunition boxes to keep out water. Thus it did not exist in 1914 when the movie is set.
As Rose is trying to climb back onto the African Queen after bathing, she is seen to be about as naked as an actress could be in a 1951 film. Her back and legs are visible. Yes when Mr. Allnut helps her get on, she is wearing undergarments of the time period of the film's setting - 1914.
Throughout the movie, Allnut goes from wearing an undershirt and long underwear to not wearing them. An example of this is when he is covered with leeches. He takes off his shirt, no undershirt. Rose raises his pants legs to check for leeches, no long underwear.
While shooting the first rapids, a close up of Rose shows her to be sitting with dry clothes. A distance shot shows water cascading into the Queen and all over, either a dummy in the miniature or a stunt double of Rose, nearly flattening her hat. In a return close up, Rose is still dry, especially her hat.
When exhausted Charlie and Rose are sleeping on the boat Rose's right elbow is folded, but when Charlie wakes up, walks on the boat and goes back to Rose to wake her up too, her right elbow is unfolded.
When Charlie Allnut gets back aboard the boat after he pulled him with a rope, just after Rose screams because she has seen the leeches on his back, the head of a member of the troupe is visible below the screen.
The propeller on the boat is made of bronze (stainless steel hadn't been invented yet). Bronze cannot be easily welded, even with the proper equipment, but he welds a new blade to the propeller. (In the book, Allnut makes a replacement blade out of iron, and rivets it to the bronze propeller.)
At the beginning of the movie, the deckhands and locals are speaking Swahili, yet receive crucial information via African drums. Since unlike most Bantu languages, Swahili is not tonal, African drums (which depend on tonality) don't work in Swahili.
Boats float because of the weight of the water that their hulls displace, but the African Queen is an open-topped vessel (no roof), so the boat could not have floated free of the mud and grass just because the rain had made the level of the river rise, since an almost equal "depth" of rainwater would have collected inside the boat as had accumulated on the surrounding terrain. So the boat would have remained at about the same "height" relative to the surrounding terrain, namely, resting on the shallow river-bottom, as it had been before the rainstorm. Charlie and Rosie would have needed to manually pump/bail the boat out before it would float any higher in the river. (However, the river was also rising upstream and so could have exerted enough force to push the boat off a patch of mud.)
Shortly after escaping the swarm of flying insects, the Queen enters a narrow channel. As it enters, water can be seen coming from an exhaust outlet on the boats transom indicating that it has an internal combustion engine and not powered by steam.
After the first rapids scene, Bogart dips a glass of water into the river, then places it out of sight. He then carefully picks up a different glass from a different spot next to him before pouring gin in and taking a sip. The water level in the second glass, from which he drinks, is clearly different from that in the actual river water dipped glass.
As 'Charlie Allnut' taunts the hippos swimming toward The African Queen, a very distinct white edge can be seen around the boiler and pressure gauge behind him. In addition, Allnut is in focus, the boiler and pressure gauge behind him are out of focus, and the trees in the distance are sharply in focus. This is all evidence of an imperfectly executed matte shot, with Allnut and boiler in the foreground image and the trees in the background plate.
Charlie at one point in the journey taunts a pod of hippos. Anyone who knows Africa as well as Charlie supposedly does would never do that as hippos are extremely dangerous and have been known to attack boats with little provocation.
Allnut gets wet sleeping under the open sky, and Rose finally lets him into the canopied part of the boat. After he falls asleep, she opens an umbrella to protect him from the rain. When she opens the umbrella, it is already wet, presumably from previous takes.
After the African Queen gets clear of the flying insects and Charlie is comforting Rosie, both characters are in focus and the engine behind them isn't. However, the trees which are behind the engine are sharply in focus. This is proof that the scene was matted and looks very unnatural.
At one point, the Queen is drifting quickly along the river and Rose and Charlie are sitting together looking toward the rear of the boat. They are not watching for anything ahead that they could crash into.
At the end, Charlie and Rose are married by the captain of the Luisa. However, their marriage would not be legal. The captain of a ship has no particular power to perform weddings. The Navies of America, Britain, and various other countries specifically prohibit a commanding officer from performing marriage ceremonies.
When the Germans arrive at the village, Rev. Sayer confronts a German soldier, who hits him in the face with a rifle butt. He falls to the ground, and the left side of his face near his mouth is swollen, bruised, and bloodied. The Germans then burn the village. A short time later, while the village is still smoldering, Rev. Sayer is working outside. Rose talks to him and brings him inside. His face is unblemished, with no swelling or bruising.
Several shots of the Luisa and the African Queen on the lake are poorly matted. When Charlie and Rosie are hiding in the brush as the Luisa passes nearby, the ship is clearly not moving forward, though it is under steam, and its size is also out of proportion to the setting. Later, in the shots of the Luisa approaching the capsized African Queen (seen in the foreground), the Queen is an obvious model and the water surrounding it blurs unconvincingly into the background shots of the actual lake with the Luisa on it.
Following the wedding aboard the Luisa, the crew began hanging Charlie and Rosie by pulling on the ropes without first tightening the nooses. As a result, the nooses began lifting off their heads when the African Queen's torpedoes exploded against the Luisa and ended the execution.