Ascended Fanon: The version of Tintin and Alph-Art by fan artist Yves Rodier almost became this, as Herge's former assistant Bob de Moor attempted to have it released as an official entry in the series. However, de Moor himself died before this could be done, and nothing came of it.
Tintin in the Land of the Soviets and Tintin in the Congo. Soviets was pretty much ripped completely from a book about Russia at the time. Hergé regretted a lot about Tintin in the Congo, such as the animal cruelty and the artstyle. He allegedly tried to get them removed from print, but at least got to assess some of his own personal issues with Congo.
One aspect of Shooting Star Herge was allegedly not very fond of was the portrayal of the antagonists as Jewish-Americans.
After Hergé announced at the end of Cigars of the Pharaoh that Tintin's next adventure was to be set in China, he was introduced to Zhang Chongren, a Chinese art student living in Belgium, who offered to consult on The Blue Lotus in order to avoid the stereotypes and caricatures typical of depictions of his homeland. Hergé accepted the offer, leading to a lifetime friendship. Zhang was even included in the story and known as Chang Chong-Chen. A humorous sequence has Tintin telling Chang about European stereotypes of China, which leads him to think that the Europeans are crazy.
Hergé was particularly meticulous in his research. He kept a huge collection of photographs, newspaper articles, and anything else possibly useful in future stories, and so many places and objects in the comic books are real places. For instance, the house of Prof. Calculus' Italian friend in The Calculus Affair is a real house that is still standing to this day. Furthermore, much of the scientific information in the books is accurate, or at least was accurate for the knowledge of the time, some apparent errors being a case of Science Marches On. In a odd case to Science Marching a 180 Degree Turn, one of Hergé's most well known "mistakes" was in depicting ice on the Moon (Explorers on the Moon), but since Indian astronomers have indeed found ice there, we probably owe him an apology.
The reason why Hergé provides no explanation for Haddock's Deus ex Machina rescue of Tintin in Tintin Land Of Black Gold (when Haddock tries to explain how he got there, he's always cut short) is that it's actually a meta joke. The original version of Land of Black Gold was initially serialized in a newspaper in 1939 and 1940, but when Germany occupied Belgium in May 1940, that newspaper ceased publication, so "Land of Black Gold" was stopped mid-story. At this point Captain Haddock hadn't yet appeared in the series, so naturally he wasn't in the original Land of Black Gold either. Several other Tintin stories were published, first in another newspaper and then in the new magazine Tintin before Hergé decided to redraw Land of Black Gold in 1948. In the intervening stories Haddock had become the most significant character in the series besides Tintin himself. Thus it would've been odd if Haddock had been left out of the new version of Land of Black Gold, but on the other hand he didn't really belong to a story that had been scripted before he even existed. This is the reason why Haddock is virtually absent from the story until the very end, and why there's no explanation for his sudden appearance. The lack of explanation is Hergé's comment on Haddock "invading" a story he wasn't originally a part of. So there is a solution to the riddle on a meta level, but not in the actual text.
Because Tintin in the Congo is often not printed that much anymore due to racism, Al Capone mentions a diamond operation in the Congo that is never mentioned again and makes the viewer wonder if s/he missed an adventure. This may seem like a Noodle Incident to the uninitiated.