For a while, a Danish Facebook page has offered for sale a copy of Tempo Tabou featuring a drawing of the Marsupilami, supposedly made by Franquin at a Copenhagen convention in 1978. It doesn’t exactly take an expert to see that the inept scribbles weren’t made by Franquin, and the piece has been widely denounced as a fake.
Once you start looking, the web is flowing over with dubious “original sketches” by Franquin and other great masters. Auction sites seem particularly prone to forgeries, as you might expect, but the images have a tendency to disappear as soon as the sale is over, leaving few samples as evidence. (Fans discuss the authenticity of supposed originals here and here, though to my eyes they can sometimes be a bit trigger-happy to proclaim something a fake.) But let’s stick to the most egregious ones:
(The last one was, I think, done as a joke.)