Revealing mistake: When the boat is supposedly surrounded by a swarm of flies, they're really just relatively few flies trapped between two panes of glass in front of the camera lens. Evidence: whenever the camera moves, the "swarm" moves with it against the background, and you can even see some of the insects walking on the surface of the glass.
Revealing mistake: When Charlie and Rosie are on the river underway, and Charlie is done mimicking animals like the hippos and chimps, they snuggle in the back of the boat, which is chugging along. The viewer has a clear shot of the water behind the boat - which has nary a sign of wake.
Revealing mistake: In some parts of the movie, the boat used is diesel powered. In one particular scene the viewer can plainly see the diesel exhaust from down low on the stern as well as small wakes on the surface of the water from the exhaust passing above it.
Revealing mistake: Charlie calls Rose to take the tiller, for the first time. She has a hamper on her lap, but there's nowhere to put it, so she takes it out and lowers it down below the bottom of the screen, then she takes her hands away and the hamper quickly disappears. Some crew member must have taken it away.
Revealing mistake: In the scene where Bogart and Hepburn are in a boat cruising down the river and laughing at some monkeys or apes on the riverbank, in the medium close-up shots of them together you can see glow and signs of matte and chromakey shining in their hair. (00:59:00)
Revealing mistake: When the explosions on the Louisa happen, towards the end of the wider shot you can see the actors stopping any reaction (Charlie pulls the head out of the noose and then just looks at Rose without moving an inch closer, and Rose herself keeps her arms down all the time without even attempting to free her neck), just waiting for their cue to do other actions in other takes. (01:42:45)
Revealing mistake: As Charlie holds Rose close and tells her that "We'll never lack stories to tell our grandchildren" after the assault of the bug swarm, the background is perfectly in focus, but the engine behind them is not - on top of showing chromakey white noise in the top right corner. (01:13:35)
Revealing mistake: When the protagonists get suddenly attacked by the mosquitoes, those appear and disappear completely randomly like in an on/off switch, both times when the boat is supposed to move very slowly or even not at all (they appear when the boat is anchored). Moreso, even if the boat is anchored, the background behind Bogart and Hepburn keeps scrolling, and when Mrs. Hepburn swats the flies away yelling "Charlie!", the contour of chromakey screen is visible. (01:10:55)
Revealing mistake: Before the boat gets into the steep rapids following the bit with Allnutt doing hippo and monkey impressions, the wide shot shows the mannequin supposed to stand-in for Bogart all bent over in a way that does not mirror the actor at that point (but fits the bit when the boat is taking a nosedive). (01:02:40)
Revealing mistake: When Charlie pours himself a drink by adding gin to a glass he filled up with river water, a little conjuring trick is at hand here; the glass Bogart spikes is not the same one he legitimately dipped into the murky river. He puts the dirty water glass out of frame and supposedly retrieves it once he opened the bottle, but it's from a different spot. More telltale signs are the difference in level of liquid, and in fact, at the beginning of the sequence once he pulls the glass back from the river, above the edge of the boat railing you could see the rim of the other glass ready to be switched. (00:40:40)
Revealing mistake: Outside the church, Allnut tosses a cigar down knowing it will attract the natives. Notice how one extra, one of the first to arrive, grabs the cigar right away, but still pretends like everyone else to be looking for it left and right with great clamor, so that the scene can continue. (00:04:35)