In the market Sakharine offers to write a check for the model of the Unicorn using a retractable ballpoint pen (you can even hear the click). Ballpoints were not sold in Belgium or the UK until December 1945 (and came into widespread use in the 1950s) and the retractable ballpoint pen was not invented until 1959. The movie takes place in the early 1930s based on the dates on the newspaper cutouts showing Tintin's reporting.
After Sakharine's car is smashed against several walls while dangling from a crane, it is shown to have the remains of broken tempered safety glass windows on the side. This is the glass which breaks into tiny segments and is fitted to all new cars, but it wasn't used until the late 1930s. As the film takes place before this, the car's glass should have shattered into large shards like a house window.
At several points during the film, characters talk about "INTERPOL". While the organization that we now commonly know as INTERPOL first came about in 1923, well before the time of the film, it did not take the name "INTERPOL" until 1956. Prior to that it was the International Criminal Police Organization or ICPO. "INTERPOL" was its telegraphic address and where it got its current name from.
A common error: in some of the fight scenes, the bad guys are using what appear to be German-made MP 40 or possibly MP 38 machine pistols. As their names imply, these were designed in 1940 and 1938 respectively, so would not have been invented at the time the action takes place.
If the film is set in the earlier 1930s then there should not have been a Citroën Traction Avant park up across the road from Tintin's flat. This car was produced between 1934-57. There are also several 2 CV Citroën in the streets. This car was produced between 1948 and 1990.
When Allan finds out that Tintin has blocked the door to the cabin where he is being held, he tells Tom to get TNT. In the following shot of Tom preparing the dynamite, he says there are other ways to open the door. But in this shot, Allan's mouth is moving and Tom's isn't.
When Tintin shows Thompson and Thomson the newspaper Barnaby was marking with blood after being injured, there is a large, round bloodstain at the end of the newspapers name. When they are outside saying goodbye, the only bloodstain in that area is a finger-thick stain of blood going downwards at about the middle of the page.
When Bianca Castafiore is about to sing, the music played is introducing an aria from Barber of Seville, by Rossini. When she starts singing, it is an aria from a completely different opera, Roméo et Juliette, by Gounod.
When the Pick-pocket picks the purse of Thompson or Thomson, he is wearing blue gloves in his hands. While running away from the duo's, when he removes his coat, his gloves are missing and same when he steals Tintin's purse.
The photographers shooting Bianca Castafiore use cameras like the Graflex Speed Graphic. While it was correct that the image on the ground glass focusing screen would be upside down, that image could only be seen if the film (sheet film holder) was not in place. In normal use, the photographer would have likely used the rangefinder and the top-mounted wire frame viewer - never looking at the ground glass focusing screen.
The photographers shooting Bianca Castafiore would have been using flash bulbs. These were single-use devices made up of a glass bulb filled with oxygen and a magnesium filament. While they are sometimes shown ejecting a bulb, they often manage to get multiple shots from a single bulb.
When the Unicorn picks up Red Rackham's ship by the mast and hoists it into the air, the footage of Red Rackham's ship swinging towards the screen begins, then cuts after barely a second and swings towards the screen again.
The goof item below may give away important plot points.
At the end of the film, Tintin is letting the gold coins fall from his hand back into the globe; in the wide shot all the coins clearly fall from his hand. In the next shot, the last of the coins fall a second time.