The Secret of the Unicorn: Part 1 - S1-E3
Other mistake: After discovering the scroll, Tintin begins reading it out. The words on the paper, the words Tintin is saying, and where Tintin's eyes move as he read the paper don't seem to match up. Tintin first says "Three brothers joined", which covers the first half of the line, but the next shot then shows his eyes reading at the very end of the line. Tintin then says "Three unicorns in company sailing in the moon day sun will speak", and his eyes are then reading across the second line, but the start of the sentence is shown on the second half of the first line of the scroll. (00:09:20)