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.
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.
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)
Chosen answer: He must have thought that Snowy had found the magnifying glass and was playing with it. Like most dogs would do with a toy.