Plot hole: In Bagghar, Allan knocks out Haddock by hitting him on the head from behind. Later, Haddock tells Tintin it was Allan who knocked him out - how would he have seen him?
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.
Revealing mistake: Mr. Silk picks Tintin's pocket when they bump into each other on the street. However, it is clear that neither of his hands go anywhere near his pockets.
Suggested correction: Have a look at the original Secret Of The Unicorn book with Herge's illustrations. Mr. Silk has picked wallets out of hundreds of people's pockets, but nobody has ever seen or felt him take their wallets. Mr. Silk must be the world's fastest and most adept pickpocket. Perhaps he's so fast that nobody sees his hands move. Did you ever see the Mel Brooks film Blazing Saddles? The Waco Kid, played by Gene Wilder, is the fastest gun in the west. Once he shoots six people's guns out of their hands. Yet you never see his hands move at all! He can draw his gun, fire at multiple targets, and replace his gun in his holster, but he moves so quickly that your eyes can't even register the movement. Maybe Mr. Silk is an equally fast pickpocket.
Continuity mistake: When Barnaby is shot, the newspaper keeps moving away/towards him between shots.
Other 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. (01:00:00)
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.
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. (00:10:00 - 00:15:30)
Continuity mistake: When Barnaby is shot and lands on the ground, his arm is touching Mrs. Finch's door mat. When Mrs. Finch steps out of her flat, the body has moved away from the door mat (without being physically moved).
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. (01:00:45)
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.