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.
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. A return close up, Rose is still dry, especially her hat.
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 can not 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.)
After the first rapids scene, there is a drinking 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 before pouring whiskey in and taking a sip. The water level in the "safe" glass, from which he drinks, is clearly different from that in the actual river water glass.
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 mosquitoes 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.
Shortly after escaping the swarm of mosquitoes, 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.
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.
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.