Sometimes, for whatever reason, a comic book
author is not able to meet his or her monthly deadline
, leaving a gap in the printing schedules for that month. Clearly, this is a Bad Thing.
A good editor, therefore, will always commission one- or two-issue scripts from other authors, to be kept on file indefinitely and dropped into schedules where needed. Such a script is known as an "inventory story".
Inventory stories seem to have fallen out of favour in recent years, with comics such as 100 Bullets, X-Men and Planetary having between one month and one year (!) between issues whilst the author gets up to speed. This seems to be a result of the modern "super star author" syndrome, where the author's fame or fansbase give the editor confidence that a gap between issues will not result in a dramatic drop-off in sales.
In years gone by, the idea that an author could leave a gap in the middle of an arc would be inconceivable. These days, certain authors are renowned for missing their deadlines on a regular basis, but are still employed by their companies. How times change.