In 1978, the then-new management of DC Comics decided to embark on an expansion plan they called "The DC Explosion." New books were created, new back-up features were written and drawn, production was hastily increased…and then suddenly, it all fell apart. In what some (inevitably) dubbed "The DC Implosion," the new projects were cancelled and a lot of completed material was "written off," meaning that they decided never to publish it. But they did publish some of it in two very-limited editions reproduced by Xerox. The two issues of Cancelled Comics Cavalcade contained little in the way of wonderful material but were so rare as to evoke mega-prices and great curiosity.
That's the short version of the story. A longer explanation has been written by my pal Bob Rozakis, who was working for DC at the time. Here's Part One of a series of columns he wrote detailing the history of one of comics' scarcest publications. And since there don't seem to be any links on that site to take you from one installment of Bob's column to the next, here are links to Part Two, Part Three, Part Four, Part Five, Part Six, Part Seven, and Part Eight. I think there are other parts beyond those but these are all I could locate.
If you'd like to see these "lost" comics, there are bootleg copies sold from time to time at conventions. Scans also seem to turn up from time to time on the Internet. At the moment, this website is displaying many of the stories, at least until some DC lawyer suggests that ain't a good idea. A few of the stories are pretty good, and the worst are no worse than lots of stuff that made it into print at the time. Interestingly, the comic from that period that seems to hold up better than any others for me is Joe Simon's Prez. The book was quite controversial at the time, inexplicably hated by a number of comic fans who didn't like that DC was wandering into the domain of humor comics. In hindsight, it may have been the cleverest, freshest thing the company published for years. The fifth issue never made it to real publication but was included in Cancelled Comics Cavalcade and demonstrates why Prez is on my list of comics that were considered flops but which I think the publisher just gave up on too soon.