Factual error: Tintin says they only have one bullet left. That bullet is in the magazine,and since it is a semi automatic, there should also be one in the chamber, meaning they would have two bullets. Even if they only had that paticular bullet, he doesn't cock the gun, meaning the chamber would be empty and therefore wouldn't fire.
Deliberate mistake: There's no way the tip of a mast is strong enough to support an entire swinging ship, even if the latter one is a lot smaller.
Plot hole: Tintin has a pretty sharp eye (as seen in his fight with Haddock, in the radio room, the Thompsons following them etc.), but he didn't notice that Sacharin's Unicorn was still fully intact? His own model was pretty badly damaged, with all sorts of wires hanging loose after its fall.
Continuity mistake: During the opera scene, the first shot of the bulletproof glass shows that there is a wire webbing within the glass. In subsequent shots, and when the glass shatters, there is no sign of the wire.
Factual error: When the Unicorn is flat on its side, Haddock cuts the line connecting it and the pirate ship. After that, the Unicorn completely resets itself to an upright position. 18th century vessels weren't built for that. The first big self-resetting ship wasn't built until 1926. The Unicorn would have capsized in that situation.
Factual error: In the scene when the opera singer arrives in Morocco, she claims it's the first time she's visited the Third World. In actuality, the term "Third World" wasn't coined until the Cold War which didn't start until a few years after the source material for this movie was written.