Gunnerkrigg Court: In this strip, Kat and Andrew's facial expressions reference a Running Gag involving Kat's love of birds, while Annie talks to herself in the foreground.
8-Bit Theatre does this occasionally with Black Mage asking White Mage out and getting beaten up.
Conversely, this strip had Black Mage asking her out with her trying to think of a proper response to get him to really stop asking her, while Fighter, Red Mage, and Thief float off into the sky seemingly because Fighter "slayed gravity".
Inverted in EarthBound fan comic The Chosen Four, thus making it a "Funny Foreground Event." While the main plot takes the back seat, two Exit Mice can be seen chatting. Meanwhile, Ness, Paula, and Jeff can be seen running around in the background in a comedic chase with the rabidly decreasing Guardian Diggers, which is then joined in by Gigantic Ants attempting to avenge their brother Titanic Ant, and then all of the sudden a horde of Mad Ducks and Noose Men.
It also happens numerous times once Doc Scratch takes over the narration. For instance thesetwo pages, wherein SS can be seen in the background (and page header) employing atrocious candy bowl etiquette. Then later, while the audience is distracted by Vriska vs. Bec Noir, SS wanders around in the header, casually spreading gasoline around the room, which he then lights on fire.
Inverted as well in that some fans actually forgot to look at the main panels, focused on the Background events, such as Jade accidently killing Dave, and didn't notice Doc Scratch as he beat up Slick with a book.
During Dave's adventures in the LOHAC Stock Exchange, various timeshifted Daves are wandering around in disguises, including one wearing a beagle pusson top of his shades.
Numerous examples in Bite Me!, such as there being hilarious meanings to french written on signs in the background, or tiny snippets of background conversations, once even between rats discussing political philosophy.
Done subtly in Freefall. During the visit to Ecosystems Unlimited arc, careful observation will show that every time Sam encounters someone with a differently colored security pass, he will steal it and replace theirs with whatever one he has at the time.
The Wotch has a kid on a skateboard who runs into telephone poles a lot. See here. He gets a slightly more important role here.
Unwinder's Tall Comics. In this strip, Howard is in the background, apparently spewing flies out of his mouth. In this strip, Moist Rider can be seen working in the background of Howard's coffee shop, which ties in with that strip's Alt Text, "Howard won't hire anyone he didn't create."
In the visual novel styled morphE all of the backgrounds in Amical's manor include paintings of Amical. Most of them are normal enough but there are some abnormalities which appear briefly in important CG scenes or distantly in the background of some shots. Examples to date include Amical skiing away from the SkiFree Yeti, Amical as a mermaid and Amical streaking.