Hmm, your mention of Seven Crystal Balls and Picaros makes me wonder if I should be looking at Alcazar/San Theodoros-related things, and your clue about the grammatical error concerning a proper name makes me wonder if you're refering to the way that San Theodoros's capital sometimes changes its name from Las Dopicos to Los Dopicos.
On page 16 of The Broken Ear the captain of The Ville de Lyon calls it Las Dopicos. But the heading on the letter tintin gets on page 17 reads: Ministry of Justice, Los Dopicos. Then the faked letter given to the ship's captain on page 19 refers to Las Dopicos again. And on page 1 of Tintin and the Picaros, Tintin refers to Los Dopicos (as the former name of what is by then called Tapiocapolis).
However, I don't speak Spanish so I don't know if Dopicos means anything, or if Las would be more or less grammatically correct than Los or if it makes no difference. And anyway, now that I check, I can't find any direct references to the city in the two Alcazar-featuring books, The Seven Crystal Balls & The Red Sea Sharks, nor in any other, so since your question asks for an error which occurs in four Tintin books, maybe this isn't what you're after at all. But that's my best guess!
Edit: Having just re-read your original question, I'm pretty sure my Los-Las answer doesn't fit what you're looking for. Never mind! |