In 1995, the ABC sitcom Full House was cancelled, with the network citing increased production costs and the contracts of several main actors expiring as the reasons for cancellation. However, despite cancellation, there were ongoing negotiations to keep the show going on the new network The WB, though these plans never panned out, and Season 8 was the final season of the show.
However, there were allegedly several scenes for a planned Season 9 filmed during Season 8's production that were to be shown to WB executives as a test to see if they would want to pick up the show. But, several main cast members (Stamos, Cameron) wanted to move on and would either be merely recurring/guest characters or not appear at all, and The WB was only willing to hire one Olsen Twin to portray Michelle, so the plans fell through. The cancellation was pretty abrupt, as the "finale" (Michelle Rides Again) was not actually written to be one, so they had to add a few new scenes to give it some sense of finality, and some of the younger actors allegedly didn't even know the show was done until the final taping (which was NOT the final episode, but another episode in Season 8).
There were allegedly some set photos in an entertainment magazine of Season 9 in 1995, but beyond that, evidence of any footage seems to be lost. It very well may never have existed, but I've seen enough Reddit posts and YouTube videos talking about it to think that there might legitimately be something out there.