Pokemon Doujinshi Incident

The page states that not only did the doujin feature Ash and Pikachu in less than favorable positions but that it also was able to be bought via mail order instead of being sold in person which made it easily accessible to minors.

Also because Nintendo and copyright, if you're asking to find it then I think it would be a hassle considering we were never given the name of the artist and even then you'd be essentially be looking for a doujin that features NSFW acts between a minor and an animal.