For those that don't know, Spider-Man made the jump from comic book to a full-length magazine for just two issues in 1968 under the title The Spectacular Spider-Man. The series was abruptly cancelled before the third issue could see release for unknown reasons.
Lost media comes into play because the third issue was well into production and advertised at the end of issue #2 under title "Mystery of the TV Terror." We don't know how much of the comic was completed before it was abandoned, but it's mentioned that Stan Lee, John Romita, and Marie Severin were still actively working on it at this time.
Unlike the previous two issues of Spectacular Spider-Man, this story was never recycled into the ongoing Amazing Spider-Man comic. This was debunked by CBR in the following article: www.cbr.com/spider-man-tv-terror-prowler/ One of the surviving production assets is Romita's design for the story's villain; which happens to be the same costume later used by the Prowler. Otherwise, the Prower's debut issue has no relation to this lost story.
CBR also mentions this comic almost being released, but abandoned yet again. I haven't seen any elaboration on that.
Just thought I'd bring this to everyone's attention; a graphic novel-length Spider-Man story conceived by creator Stan Lee and definitive artist John Romita is abandoned in an unknown state. Could we see it finished off and released someday?