Clue From Ed

Comics trope for any long-running serialized story with an intact continuity. An event mentioned in dialog or captions has an asterisk next to it, and down near the panel border is a little message, saying something like, "See Volume 4, Issue 3 - Ed." Sometimes, in shared universe titles, it may be an issue of a completely different comic series.

Some comics fans, especially young ones, may wonder who this Ed, the enormously helpful man at the comics company, must be. Of course, most of the older fans figure out that this is in fact "the editor". This little footnote also entices the reader to seek out the back issue that includes the story in question. Or, if they don't have the issue, it's an incentive to go out and buy it.

Back in the Silver Age, many of these footnotes in Marvel Comics would be credited to a "(Adjective that begins with 'S') Stan". This was, of course, a reference to Stan Lee.

Use of these notes has been dissuaded by companies today (and they are allegedly all but banned at Marvel to "hide" the Continuity Snarls), but they are starting to make a comeback.

Can also be used for a joke in an article, in which case it's Note From Ed.

Not to be confused with a Signal from Fred (an Evil Twin of Lampshade Hanging identified in the Turkey City Lexicon) or the (slightly) more literary Note From Ed.

Examples of Clue From Ed include:

Comic Books

Subverted in the first issue of Wisdom, in which the clues all refer to comics that have never existed.
- The Infocom computer game Leather Goddesses of Phobos came with a "Lane Mastodon" comic that did the same.
- Megaton Man did this as well, referencing to titles that never existed.
This photoshopped page (originally from Civil War) starts off with a very good example of how editorial footnotes used to be used, and then goes on to parody Marvel's current-day reluctance to use them as fictional versions of Mark Millar and Joe Quesada enter in their own editorial boxes to try and throw Stan Lee's old-school notations out (and him).
DC Comics blundered themselves into a minor flackstorm of criticism regarding editorial footnotes. When asked why Countdown, which was absolutely dense with references to events from other comics to the point where it was pretty much unreadable on its own (okay, it was pretty much unreadable, period), had no editorial footnote boxes, DC responded that "that's what the Internet is for". The Internet was not amused. DC started putting the boxes in.
Used very sparsely in Hellboy, generally to point you towards a cool event that was just mentioned. They are not attributed to an editor.
Used a lot in the Sonic Comics, even using it as a method of MST in some parts.
Done in Walt Disney's Comics and Stories #699, as it's Boom's first run of the series which had been not running for about a year, and dealing with Disney Superheroes that American audiences wouldn't be expected to be familiar with (except maybe Super Goof). They decide to turn it into a running gag, pointing out things that should be really obvious. (hence the name "-Obvious Aaron")
The Amalgam era of Marvel/DC used these, but more often than not subverted this by referring to comics that didn't exist.
2000 AD's Editor-In-Chief Tharg the Mighty always refers to these as "Tharg Notes."
Often parodied in Deadpool comics: for example, when a character makes a reference to something that happened during the Onslaught event, the clue box refuses to admit it ever happened. Another time the box blatantly admits that there's no good reason to remember the villain from an earlier appearance in another comic, as it wasn't very good.
Frequent occurrences in Tintin, and quite helpful at that since all the stories have names, and therefore it's easy to recall in what context the character/event/general happening last made an appearance. Of course, it also helps that there are only 23 albums.

Literature

One set of House of Leaves' footnotes is of this sort.
Some of the old Doctor Who Novelizations have footnotes explaining Call Backs and Continuity Nods.
The second and third James Bond novels, Live and Let Die and Moonraker, have footnotes for the first reference to the previous novel.

Manga

Shonen Jump tends to do this to explain cultural terms or Japanese writing. One recent issue of Ultimo got stupid with it, though, when a character said he was a bodyguard of the nobility, and the footnote explained that bodyguard meant "a bodyguard of the nobility".

New Media

Used in the "iFanboy" podcast to refer to earlier episodes or make corrections to misspeakings by the hosts.

Web Comic

Parodied in a Penny Arcade comic about Tycho's hatred of the undead. The print version takes it a little farther, referring to the strip's "Kool-Aid thin continuity".
Parodied on this page of The Adventures of Dr. McNinja.
- Also here.

Honestly, why haven't you read all of D.A.R.E. by this point? I've given you plenty of notice. What is wrong with you? - Ed

One Narbonic strip had these in every panel. The first panel of the next strip had one which Helen interrupted with a minor fourth wall breakage.
Gunnerkrigg Court: Tom Siddell made this subtle piece of satire as a response to the fans who were confused by the Flash Back on the original version of page 437.
Parodied in this Dinosaur Comics strip.
Axe Cop links some seemingly out-of-place facts to the Fourth Wall Mail Slot or printed media.

Web Original

During the various Global Guardians PBEM campaigns, this was used quite often when game masters had to reference events that occurred in their campaigns years prior, sometimes to players and characters who just weren't around when those events occurred.

Real Life

Chick tracts do this with Bible verses. They don't always have a clear connection with the events of the panel.