A variation of this trope is often run into in Journal Roleplay games that use "fandom" characters. Many games have a "no fourth-walling" rule — i.e. one character can't reveal to another character that they're fictional, or use knowledge of their original canon to their advantage.
On the other hand, there are pan-fandom roleplays that are more lax about the Fourth Wall, usually with the bottom line being "but only if the player of a character is okay with it". (However, even the most meta-aware No Fourth Wall character is still not allowed to recognize that they're in a Journal Roleplay regardless of premise or setting.)
In the third episode of Amazon's Betas, a background character in a karaoke bar sings "Gives You Hell" by the All-American Rejects. That band's lead singer Tyson Ritter has a recurring role, not as himself but as a drug dealer; in that episode's B-plot, the other characters bail him out.
Brad: ...So the Olsen twins exist in this universe! What are they famous for, New York Minute?
In Carmilla the Series, Sherman Hollis is played by Enrico Colantoni. Colantoni also played Keith Mars, Veronica's father in Veronica Mars, frequently referenced in Carmilla.
Kamen Rider Ex-Aid: The Abridged Series has Kuroto and Parad trying to start a web vlog called "Bugster Bros", only to learn the name has been taken by SpecialForm12 and ProtoDubs, the YouTubers who voice the characters.
In the KateModern episode "Fictionality", Ralf Little's character, Gavin, complains that people keep accusing him of being a professional actor.
Both averted and not in The Lizzie Bennet Diaries. The vlog that Lizzie does is available in-universe and is clearly relatively successful. Most characters are aware of it. On the other hand, none of the characters has ever heard of Pride and Prejudice.
However, Jane Austen seems to exist, as Charlotte and Lizzie mention that their mothers were reading Sense and Sensibility at a book club when they went into labor.
When Lizzie bashes Darcy's name, Lydia mentions that Colin Firth (who Lizzie adores) once played a character of the same name. If Pride and Prejudice doesn't exist, what was Bridget Jones based on? Does it still have a similar plot?
New York Magician: Discussed when Michel meets Cthulhu. He's sitting in a bar later on, more than a little disturbed, and points out to the djinn he's discussing the incident with that said Elder is supposed to be fictional. The djinn's response is basically "That doesn't mean he isn't real."
On NoPixel, Carl Crimes is an Expy of Carl Grimes from The Walking Dead, and is played by Carl's actor Chandler Riggs. However, The Walking Dead also exists in-universe, and no one has asked why Carl Crimes looks exactly like his television counterpart, though it's never been definitively determined whether Riggs himself exists in this universe.note That said, it is worth noting that the cops used a picture from the TV show as Carl's mugshot, so it's anyone's guess at this point.
Noob plays with the trope both in terms of the series itself and stuff related to it. Noob-related objects can bee seen in Fantöm's room and the channel that airs the webseries on TV exists. Omega Zell has a ringtone that is none other than his own character song and a shot of the band that performed it is briefly seen between two TV ads related to the Fictional Video Game of the setting. The lead singer of the (real) band is the actor playing Master Zen, a character that is a hiding escaped prisoner.
In the TGWTG Team Brawl special, the Nostalgia Critic and Chester A. Bumare both in the fight, and then "Ask That Guy" comes in near the end and gives the entire brawling mass a lecture about misdirected creative energies, and proceeds to take a group photo.
And this was also lampshaded when the three characters all explained how they ended up shaving their beards at the same time.
Don't forget the April Fool's gag where the three characters rotated sets for a video, so that the Nostalgia Critic became severely traumatized by all these horrifying questions he was getting from the audience while That Guy rambled on incoherently (in his same old monotone) and then demanded change, and Chester reviewed an old movie.
In an exclusive video on the Volume 2 DVD, Ask That Guy is asked why he, the Nostalgia Critic, Chester A. Bum, Dominic, Black Dog Bill, and General Zod all look alike. Rather than give a "wacky in-universe" explanation, Ask That Guy just breaks the fourth wall. "Because we're all the same actor, you idiot!"
Lampshaded in Obscurus Lupa's Charmed reviews. One storyline involves Phoebe dating a character whose actor is also on Sex and the City. As a gag, at one point she holds a DVD box for Sex and the City and compares it to her own life. Lupa then questions why she doesn't notice that one of the show's characters is identical to her boyfriend.
Lampshaded during the riffing of The Night Dracula Saved The Earth by RiffTrax, when a suspiciously familiar character steps out of the mist.
It is extremely obvious that thanks to Like Reality, Unless Noted, Marble Hornets exists in the universe of The Slender Man Mythos. It is not a stretch to assume that everything that has anything to do with the Slender Man at all exists in said universe. Which means the Slender Man stabbings had actual Slender Man involvement. Which means, considering the TulpaTheory, we'reall fucked.note In the case you didn't click either of those links, the Tulpa Theory means, in a nutshell, if enough people believe something exists (or can exist) it will exist. Which means if those two girls can convince people, or if enough people become deluded...
In the Netflix original Christmas SpecialA Very Murray Christmas, Bill Murray stars As Himself, but a few of the celebrity guests are portraying characters. This gets interesting when these guests happen to be people who've worked with him before. The most prominent example is Jason Schwartzman's character Elliott: Since they're both part of Wes Anderson's Production Posse, Murray has frequently been cast alongside Schwartzman, so he should find this groom who happens to be stranded with him in a hotel lounge during a blizzard strangely familiar-looking. It's not quite as glaring, but Amy Poehler playing Liz also fits, since Bill Murray has appeared on Parks and Recreation.
A meta-example appears in an online PSA featuring the former cast of The West Wing in-character to discuss non-partisan voting and campaign for Bridget Mary McCormack, who was running for election at the time. In introducing McCormack, Josh notes that she's the sister of Mary McCormack (who played Kate on the series) as one of the interesting facts about her. This prompts Josh, C.J. and Toby to briefly wonder exactly who Mary McCormack is. Then Kate shows up and remarks that whoever "Mary" is, she sounds pretty delightful, smart and hot.