If you search on eBay for Yongnuo products – such as their YN-series flashguns or RF-series radio triggers – you’ll find a number of their listings headlined “for Nikon D400”. This is not just at the Yongnuo official store, but at a range of different resellers.
The D400 DSLR camera from Nikon has neither been released nor announced. So, is this a typo, or do the people at Yongnuo know something we don’t?