Must Have Lots of Free Time

Subverting the Competence Zone results in one problem. It can be hard explaining why someone much older or younger would follow the cast around in the first place, especially if the story is trying to be realistic in explaining it and the audience immediately assumes they should spend their free time with people their own age. This leads to the reuse of characters, especially as repurposed (and often self-imagined) guardians for characters who didn't have them in the first place. If it's an adult female she's usually unmarried.

Less likely to be lampshaded in a fantastic setting, or if said folks are inhuman or magical in nature. Most of the time the audience doesn't really want an explanation.

Anime

 * Azumanga Daioh has two unmarried teachers who are on friendly enough terms that they follow the girls on vacation. This is probably an exaggeration of the closer relationship Japanese students have with their teachers.
 * There's also that Yukari and Nyamo do have a lot of free time -- Yukari is such a Jerkass that nobody except her best friend Nyamo and her students will voluntarily associate with her, leaving both her and Nyamo with absolutely no social life outside school.
 * Like the Kuroi-sensei example below, Nyamo has free-time because she's single at an age when she really should be married already or getting ready to be wed (at least by Japanese standards).
 * Likewise, Lucky Star has two such adults, who are used whenever the girls take a driving trip. Kuroi-sensei even manages to invade Konata's online gaming time during the evenings. Likewise, Konata's cousin Yui is around the girls enough that Kuroi assumes Yui has to be unmarried, since we don't see her husband (on business trips) around at all. Konata wondered if that is why Kuroi-sensei became a Christmas Cake.
 * Mai Kujaku/Valentine of Yu-Gi-Oh! (who, as a professional duelist, does have quite a bit of free time on her hands) hung out with True Companions eight to ten years younger than herself and fell in love with one of them. Then she joined a Cult. Then she suffered a disastrous marriage to - wait, sorry, that was someone else.
 * Brock from Pokémon has fallen into this. He originally joined Ash in order to pursue his dream of becoming a Pokémon Breeder, but since returning to the cast in Johto, has done almost nothing to further this goal and has passed up many chances to travel on his own.
 * However, he left the main cast at the end of the Diamond and Pearl series and decided to become a Pokémon Doctor instead, inspired by an experience he had with taking care of sick baby Pokémon on a ship. He has also served as the "doctor" of the group for the majority of his time traveling with Ash and friends.
 * Bronco Dino. He's is supposed to be the boss of his own family, but he helps out Tsuna often. One of his men even called this trope by name one time.

Film

 * Will from the movie About a Boy has an incredibly large amount of time on his hands. Justified in that he has this time because he lives off royalty checks for a popular, fictional, christmas song his father wrote. Partially deconstructed because he has so much free time, he has no idea what to do with his life.

Live Action TV

 * Ned's Declassified School Survival Guide: Since Gordy the janitor always slacks off at work, deciding he'll "let the night guy get it," he has plenty of time to hang out with the Three Amigos and other students, and help with simple plans. And obsessively chase that Weasel Mascot.
 * Buffy the Vampire Slayer had a half-decent excuse for why Giles hung around three of the kids in school--they were a freak (Buffy), a dork (Xander), a nerd (Willow), and he was the librarian; where else where they going to hang out? Plus, he was Buffy's Watcher and therefore obligated to watch her. Then, the school blew up. Having also been fired from the Watcher's Council earlier in season 3, he suddenly found himself with a lot of free time in season 4.
 * And by the point of season 5, when he got gainful employment again, it didn't need explaining, as the show kept the True Companions concept mostly alive.
 * The Janitor from Scrubs, even though he's mentioned that he has a family (his father appears at one point).
 * Correction, a man appears at some point.
 * He got a Girlfriend in the later seasons.
 * A real one. Supposedly. Maybe.
 * Now a wife. Who was attracted to him because she's a germaphobic.