Land of Black Gold: Part 2 - S2-E11
Revealing mistake: When Tintin and Oliviera da Figueria are talking with the guard at the castle door, the guard sneezes and Oliviera takes out a handkerchief saying "Some beautiful handkerchiefs." When Oliviera finishes taking out the handkerchief, as he says "Some", a weird mark appears on the screen, right where the handkerchief is located. This is visible for only one frame but it can still be noticed. (00:32:00)