Alice and Bob are together and Alice needs to change clothes for some reason. Bob turns his back to allow Alice some privacy. However, either by accident or by design, Bob finds himself facing a reflective surface that allows him a view of Alice as she undresses.
Aside from fanservice, a scene like this can serve several purposes. It can be used to ramp up the Unresolved Sexual Tension between two characters. Whether Bob chooses to watch or look away can tell the viewer a lot about Bob's character. And if Bob is unaware of the reflection but Alice turns around and sees it, it can result in Bob becoming an Accidental Pervert.
Compare Sexy Silhouette, where backlighting casts a silhouette of a character undressing on a screen.
A superpowered variation occurs in Superboy #10 in the New 52: Superboy calls out that he's started their campfire, but when she doesn't answer, he searches and is stunned to walk in on Wonder Girl (Cassandra Sandsmark) bathing in a lake. Wonder Girl angrily calls him a pervert and tells him to turn around. Superboy turns around, but puts his hand on the ground and smiles. Wonder Girl remembers that he can "see" through what he is touchingnote kind of like Toph from Avatar: The Last Airbender and angrily throws a rock at the back of his head to get him to cut it out.
In the erotic graphic novel Teach Me Too by Enrique Villagran, as part of a plan to seduce her nephew, Daphne starts deliberately pleasuring herself in a position in her bedroom where she knows she will be reflected in the window in her nephew's bathroom so he can see her. The plan works.
In Spy Boy, the titular character is sharing a plane with Bombshell (who is, as her name suggests, a bombshell). She warns him not to look back, and so he promises not to turn around while staring at her reflection in the sheer surface in front of him. His Super-Deformed alter ego informs him that he's going to hell for this.
In Grimm #2, Nick finds himself unable to tear his eyes away from the reflection of Maya (a female Grimm) undressing, mostly because she is continually dropping interesting tidbits as she speaks, demanding his attention.
In Captain America V3 #9, Sharon Carter watches Bucky Barnes (who was Captain at the time) changing into his uniform in a mirror.
Bucky: You're not watching me, are you? Sharon: (staring directly at the mirror)Nope.
In G.I. Joe: Retaliation, Flint watches Jaye change out of her evening gown in the reflection on a TV screen, which hints at their relationship in the comics where they are a couple.
In Dear John, John watches Savannah undress by her reflection in the mirror.
Used in Raiders of the Lost Ark: Marian is given a dress by Belloq, she goes to the back of the tent and Belloq happens to have a mirror where he can see her undressing, which further marks Belloq as a creep.
Happens in the Flash Gordon movie. In the scene after Flash is revived, Princess Aura tells him to change into a noble's uniform and turns around. However, she uses a mirror to watch Flash while he undresses.
Dan in Real Life: Dan is hiding in the shower when Marie starts to undress. He tries to avert his eyes, but ends up looking at a reflection of her.
In the 1990 film The Great San Francisco Earthquake, there was a scene where a husband and a wife were in an elevator and the wife needed to change. She told him to turn around and he did while the wife stripped her top off. There just so happened to be a mirror in the elevator and the husband glanced over at the mirror as the wife changed. No nudity was seen by the audience, but we can tell by the husband liking what he saw.
The Parole Officer has socially inept Simon Garden accidentally seeing his proto-girlfriend naked after her pet cat shoves the bedroom mirror to the correct position.
Happens in a Judge Dee story. The judge's lieutenant is sent to interrogate a girl, who tells him to turn around while she's changing. He complies, and she turns around, clothes dropping to the floor. She then tells him he can turn around now, but he replies that the mirror in front of him does the job just fine. A Sexy Discretion Shot ensues.
In the Xanth novel Demons Don't Dream, Nada Naga insists to Dug that he must not try to see her naked. When she changes in one scene, he attempts to sneak a peek at her anyway with his belt buckle, and the "computer game" that he's playing to connect him to the world of Xanth ends up booting him out because of it.
Similar but not identical: In an episode of Threes Company, Jack & Chrissy are temporarily sharing the bedroom normally used by Chrissy & Janet, but Janet is afraid of a mouse in the room. Chrissy tells Jack to turn away so she can get into bed, so he turns and looks right at a mirror, which shows him Chrissy in her skimpy nightie.
Hollyoaks: After Newt accidentally soaks Theresa with a bucket of water, he offers her his jumper to wear. As she removes her wet top, she tells him to turn around, which he does, however watches her in a reflection on a TV. This is an early indicator of a later hook-up between the two characters.
One killer on Law & Order: Criminal Intent was a voyeuristic doctor who strategically placed a stainless steel cabinet door in his office so he could watch his patients change in the reflection.
In Pretty Guardian Sailor Moon, Mamoru Chiba sees Usagi transform into Sailor Moon while watching Usagi's reflection in an amusement park mirror maze.