Former comic store employee (with near-psychiatric tendencies). At hand is the legendary error-edition of Sandman #18: Dream of a Thousand Cats.
Backstory: For the initial printing of this issue, a mistake was made in that several panels on the first page were printed in blue (versions the intended colour of yellow). The error was caught quickly after release and the botched issues were recalled.
But not before a handful of them made it to comic shops. Subsequently, "Issue #18 Error-Edition" has become widely regarded as one of the rarest comics in existence. According to the CGC census, there are only 75 graded copies of the Issue #18 Error-Edition....
And of those, only 21 are graded 9.6, 16 are graded 9.8, and none are graded higher. And last but not least, some CGC 9.6s have stress-lines. And this one doesn't.
It's a high 9.6. Also noteworthy: This Eisner Award winning issue also represent a high water mark for Gaiman's wildly imaginative and groundbreaking writing for the Sandman.
And as such, is a definite fan favourite. Altogether, given that the Sandman is now widely considered as one of the greatest comics series of all-time.
As well as its (and Gaiman's) cult-like following.. This issue promises a massive longevity for being sought after.