Cigars of the Pharaoh: Part 1 - S1-E6
Revealing mistake: The shot of the letter being placed into the envelope and being stamped with the symbol is used twice. The first time is after Tintin meets R.J. Rastapopolis. The second time is after the boat arrives at the docks and Tintin goes onto another boat. Notice how the way he closes the envelope is the same and the stamp of the symbol is placed in the exact same position. (00:03:30 - 00:05:05)