Author Existence Failure: Hergé died partway through his work on Tintin and Alph-Art; the unfinished draft has been published as part of the regular series of Tintin albums.
Creator Breakdown: Hergé wrote Tintin Tintin In Tibet largely as a sort of therapy, to resolve the emotional issues he had following his divorce and the distressing dreams he'd been having that involved vast white landscapes. It is widely considered to be his masterpiece. However, it caused Hergé to undergo another Creator Breakdown — after the book was released, he decided that there was no way he would ever write such a good Tintin story again, and effectively gave up trying to do so. The remaining three Tintin stories were released at a far slower rate than the previous ones, and took the form of experimental character pieces which tended to mock the characters Hergé had been writing over the previous decades. Tintin and Alph-Art might have seen a return to the more traditional storyline, but we'll never know since Hergé sadly suffered Author Existence Failure with the story only half-finished.
Fan Edit: At least three major attempts were made by fan artists to finish the story:
The first version, by Serge Brouillet, effectively just finishes up the work that was already done on the story, and adds in an abrupt Downer Ending where Haddock and the Thom(p)sons break into the villains' workshop, but are too late to prevent Tintin from being turned into a statue.
The second version, by "Ramo Nash," expands the story out to the full length of a Tintin album, and reveals that Akass is indeed Rastapopoulos, with Allan also briefly showing up. While a more complete story, it suffers from Wall of Text dialogue, and the entire last third of the book essentially just being Tintin and Haddock making their way home to Marlinspike, with Rastapopoulos being foiled off-page by the Thom(p)sons.
The third version, by Yves Rodier, is the most well-known edition and nearly became Ascended Fanon, though this ultimately fell apart due to legal issues. This version is treated as a Grand Finale to the whole series, with Rastapopoulos appearing again — though in this version Allan only appears in a one-page cameo, having pulled a Screw This, I'm Outta Here! — and finally meeting his end at the story's climax.