This has happened to me a couple of times, the first was several years ago when I bought my Sony Lasermax LDP-1500P which still had a Pink Floyd laserdisc in when it arrived, which I think was one disc of a two-disc set.
More recently, I bought "Harry Carpenter's Videobook of Sport Volume II" late last year, a 2-disc CAV disc, but only disc 2 was present when it arrived. I tried to ask the seller to search for disc one but there was a bit of confusion with the seller thinking that by "disc 1" I meant volume 1, a totally separate release, so I didn't get anywhere.
It didn't matter though, by one of those curious concatenations of circumstances, I'd bought another copy from another seller in the same week, as part of a job lot with 2 further copies of Volume I, which I already had! So now I've got 3 copies of Volume I and one-and-a-half copies of Volume II!