Journal de Spirou #4078 (8. June, 2016) is actually the four thousandth issue of the Journal (because when the magazine adopted continuing numbering instead of within-year numbering after the war, they simply counted the number of weeks since the first issue, even if no magazine had come out that week). For this occasion, the magazine features a selection of “scoops” from the history of the magazine, as told and illustrated by Fred Neidhardt. This includes a crossover between Spirou and Tintin!
Tintin and Spirou were published in competing magazines each named after the character. In 1965, the two rivals collaborated on an April Fools gag where they “switched covers”, each adapting the style and layout of the other:
The story goes that Franquin and Hergé wanted to take the joke further, creating a crossover one-pager in collaboration. However, the idea was shot down by the editors of the respective magazines, and the pencil sketches have remained hidden in the Dupuis archives until now:
Of course, this is another of Neidhardt’s little hoaxes. The Spirou drawings are copied from various Franquin albums (you may recognize the pose in the “Hands up!” panel from QRN sur Bretzelburg, for example), and the supporting documentation is all faked (including a video clip where they supposedly talk about it, but which is very clearly overdubbed).
Still, it got us a Spirou–Tintin crossover, even if it had to fake it!