Factual error: In the scene where the German East African troops are shooting at Bogart and Hepburn, their rifles are Gewehr 88 "Commission Rifles'" with five-shot magazines. In actuality, the German East African troops, under the command of Colonel Paul Von Letow-Vorbeck, were equipped with the absolute dregs of the Kaiser's arsenal: Gewehr 71 single-shot rifles, and Gewehr 71/84 8-round tube magazine rifles whose magazines usually got fouled and had to be used as single-shot rifles anyway.
Factual error: The scene where the German officer is blinded by the sun and thus can't shoot at Humphrey Bogart could never have happened. The Imperial German East African Army had no rifles capable of accepting a telescopic sight. In fact, throughout the war, its commander, Colonel Paul Von Letow-Vorbeck, had his marksmen trained to use open sights, not scopes. The rifle used in the scene was a British Lee-Enfield No. 1 Mk. 1 modified to accept a telescopic sight, which would have been scarce in Africa, even in the British Army. But it couldn't have been captured, as unit armorers in the British Army began scoping Lee-Enfields in mid 1915.
Factual error: During the last scene, after the German gunboat sinks and the two main characters are thrown into the lake, both Bogart and Hepburn are swimming toward the shore of the lake. Even though there is nothing which could reflect the sound of the two swimming and splashing water, one can hear the echo of water splashing. Apparently, the last scene was filmed in a pool on a sound stage.
Factual error: This comes directly from the novel, but the Ulanga river the protagonists are traveling by boat to reach the lake with the German battleship, does not in fact encounter or form any lake in its path. It also travels towards northeast, so the opposite way of what the map shown in the movie indicates. (00:22:00)