Anime Tenchou x Touhou Project (partially found OVA; 2010)
Touhou Project is a large series of shooting bullet-hell video games developed by Team Shangai Alice, known by being developed by only one person called ZUN. The game is very acclaimed in the indie Japanese sphere and has inspired many derivative fan-works like music, comics, and animation. On the other hand, Anime Tenchou is the mascot of Animate Ltd., the largest retailer of anime, manga, and video games in Japan.
In some point of 2010, Ufotable, the anime studio responsible of Fate/Zero and Kara no Kyoukai, announced a crossover special featuring Anime Tenchou and Touhou Project. This was to celebrate the 10th anniversary of the said mascot and was expected to screen at the 2010 Animate Ichioshi Bishojo Anime Matsuri.
Due to the strong restrictions on derivate works of Touhou Project, fans were surprised about the announcement by a commercial studio. ZUN, the creator and unique developer of the games, has little confidence about commercial video game distribution. Because of that he only allows that fan-works are made by fans for fans, with any intromission from corporations. And above all, with his permission.
On November 20, 2010, Ufotable released the OVA at Ufotable Café as a private screening. The use of cameras and another devices was prohibited and some reports that the place had max security. Three days later, Animate announced through their website that another screening would be carried out but with a limited number. They also announced a card set based on the OVA.
At the end of December was announced the release date of the DVD: January 7, 2011. It would cost 1500 yens and would be acquired through the Animate website. However, until today the OVA hasn't resurfaced on the internet, or even known if the DVD existed.
In October 2015, a worker from Animate and Ufotable named as Koyama confirmed through the Machi★Asobi live stream the re-release of the DVD. Even stated that the OVA would be screened on October 11 in some Japanese cinemas. But after that, nothing was heard again.
According to some tweets by Japanese people, the OVA has been screened in some events where Ufotable was present. 
On July 19, 2020, user Final斯坦 on BiliBili posted a video of the full storyboard for the film. In the video, existing screenshots from the OVA were used for references. 
- Anime News Network article, about the announcement Retrieved 22 Jan '18
- Animate website, announcing another screening Retrieved 22 Jan'18
- Komaya worker statement Retrieved 22 Jan '18
- Japanese tweets acclaiming saw the OVA Retrieved 22 Jan '18
- The full storyboard for the OVA uploaded onto BiliBili Retrieved 12 Mar '21