Forgotten Anniversary

The wedding anniversary is very important to married couples - it signifies that they acknowledge how long they've been together and how far they've made it. It's like a marriage's birthday.

Woe befall the one who forgets the anniversary.

If one member of a couple forgets an anniversary it's considered a very big problem for the relationship (at least in fiction). Someone who forgets his/her wedding anniversary is either considered insensitive enough to not value the importance of a long-lasting marriage or unromantic in regards to not putting the marriage over his/her bad memory.

There are many variations on this theme:

One of them forgets. The other sees it as pathetic, but nothing major happens, even though the other may be bitter or prone to teasing the one who forgot for a while.
One of them forgets the anniversary and the other is so mad that he/she storms out of the house to go drink off the sorrows/storms out of the house to go back to mom/mope about it in frustration. This continues until the one who's forgotten it improvises a gift or improvises something.
One of them seemingly forgets the anniversary and the other gets mad - but as it turns out, he/she hasn't forgotten! It's supposed to be a surprise gift or party, but now the one who's "forgotten" must deal with the ire of the other person...
One of them forgets and is forced to do stuff by the other's whim in order to "make up for it".
They both forget and later realize how pathetic they both are.

Oddly enough, in the one-person-actually-forgets scenario, it's usually the guy who forgets. Make of that what you will.

Usually (but not always) played for comedy.

May lead to some Convenience Store Gift Shopping. For a non-marriage example, see Forgotten Birthday.

Examples of Forgotten Anniversary include:

Advertising

A credit card commercial where a man at work is asked by his wife if he made any plans for their anniversary. Naturally he must scramble for an Ass Pull: "Uh, yeah...we're... " *glances at Chinese takeoutbox* "taking the Orient Express to..." *glances at stained tie* "a spot in...Thailand!" *glances out window, where a bird lands* "We're going birdwatching!" At the end of the commercial, on the trip that his credit card company was able to drum up on short notice, his wife remarks that "this is such a surprise", and he wholeheartedly agrees.
Another commercial had a guy's wife cook a fancy dinner, dress up, and light candles. Her husband then comes in, takes it all in with a wow, and asks "What's the occasion?" His wife then takes the steak back into the kitchen, probably to throw it away.
Another commercial (This one for the latest iPhone) has the guy at work reminded of their plans by his wife, and making a dinner reservation while he's talking to her.

Anime and Manga

In an episode of Wedding Peach this is the central problem; Hinagiku's mother attempts to leave the house because her father forgot, but as it turned out, the father was too busy with work and had hidden his anniversary gift so he couldn't present it to her the whole day. The girls try to get him to reiterate what he wanted to say in front of her, but then demons intervene and stuff tries to get in the way. They work it out.

Film

In the movie Family Man, when the main character forgets his anniversary, he forgets it because he hasn't technically been married to his wife yet, but she doesn't know that...
One scene in The Sixth Sense looks like this, with Bruce Willis showing up late to an anniversary dinner with his wife at a fancy restaurant; she barely acknowledges him before leaving. Subverted, though, because he's actually a ghost and she can't even see or hear him. It does however say very little good about the state of their marriage that it takes him 'til the end of the film to realise this.
In the 1949 Milton Berle film Always Leave Them Laughing, a guy buys a watch for his wife, and she has no idea what the occasion is. He lampshades the usual nature of the trope by saying something to the effect of, "Folks, you're witnessing a historical event. Usually, it's the man who forgets the first anniversary." His wife feels bad about it for a moment, but then gives him a kiss for always being so good to her.

Literature

In Atlas Shrugged, the fact that James Taggart can't remember his wedding anniversary contributes to his Villainous Breakdown.

Live Action TV

Nearly all TV sitcoms involving a married couple will fall back on this at least once in the run of a show. And it is by far more common for the husband to be one who either forgets or is pretending to forget.
In the Fawlty Towers episode "The Anniversary," Basil Fawlty plans a surprise anniversary party for Sybil, but she assumes he has forgotten and storms off, leaving him to try and maintain a facade of normality in front of the party guests.
In I Love Lucy, Lucy thinks Ricky gave her a mink coat, and says she thinks it's an anniversary gift. At the end, this was subverted when it turns out she lies about that, to the point of forgetting their real anniversary date, in order to get things from him.
Leo McGarry does this very early in The West Wing.
Sorkin likes this one. It's played with in one of the first season episodes of Sports Night: Dan is pissed off at Casey for forgetting their "anniversary" (the date of their first show together).

Dan: I remember what you were wearing. Do you remember what I was wearing?
Casey: I remember not thinking at the time that you were a woman.

White Collar: In the pilot, Peter has an embarrassed moment when Neal remembers his anniversary before he does. He then spends the rest of the episode desperately trying to think of a gift for his wife that will make up for having forgotten last year.

Music

The subject of the "Anniversary Song" by Da Yoopers. The wife asks her husband if he remembers what day it is, and he guesses "trout season?" and later her uncle Ernie's funeral, where she got sick. In the second verse, she fills in the details a bit, but the only thing he remembers is someone in the congregation passing gas.
In one children's song, the singer's mother gets back at her husband for forgetting their anniversary by giving the children a bowl of ripened fruit. The father sits at the low end of a table on a slope, and Hilarity Ensues when the fruit rolls onto him.

Video Games

Rose's Never Live It Down moment in Metal Gear Solid 2: Sons of Liberty is her continued "do you remember what day this is?" nagging. Apparently remembering anniversaries is Serious Business compared to saving the President of the United States from execution by terrorists.

Web Comics

Spoofed in the webcomic Pv P. The author admitted (via twitter) that he'd completely forgotten about characters Jade and Brent's first anniversary, so didn't do a strip for it. The next day, he started a storyline about how Brent and Jade had themselves both forgotten the anniversary.

Web Original

On The Lot had a short film called "Sweet" where the man makes an epic effort in getting last-minute plans together for his anniversary. Hilarity Ensues.

Western Animation

In the Phineas and Ferb episode "Dude, We're Getting The Band Back Together," Lawrence Fletcher forgets his wedding anniversary, and his wife is angry with him. Phineas and Ferb help their dad make up for it by duplicating the rock concert at which the couple had their first kiss.
Mung Daal from Chowder does this.
Cosmo tries to leave the fishbowl in an episode of The Fairly OddParents when he believes Wanda is angry at him for no reason and has forgotten their anniversary. Turns out, it was the False Anger Anniversary (once every someodd years) and Wanda had remembered, but it's too late - he's gone home to his mom. They work it out anyway, only for Cosmo to forget it was their anniversary at the end of the episode.
In The Simpsons , Homer Simpson does this several times.

Marge: "He forgets [all] holidays, both religious and secular..."

There was an incident regarding this in Danny Phantom - Danny had lost the gift and he had to go retrieve it in a ghost world. It's revealed that Jack has forgotten his and Maddie's anniversary eighteen years in a row because he's so obsessed with ghost hunting.
In The Flintstones, Fred often had difficulty remembering his and Wilma's...except whenever it fell on Trash Day.
A double example in American Dad:

Stan: This midnight fishing is great.
Bullock: Sounds to me like someone doesn't want to go home.
Stan: Yeah, I'm, uh, laying low. Today is the anniversary of a huge fight me and Francine got into last year.
Jackson: Yeah? What about?
Stan: Oh, I forgot our anniversary. Huh! I'm never gonna do that again.
Jackson: Um...?
Bullock: (shushes him)
(Stan takes a sip of his beer, casts his line, pauses a few seconds)
Stan: AAAAAAGH!
Bullock: There it is.

In Father of the Pride, one year prior, Larry had promised to never forget the wedding anniversary ever again. Guess what happens.
In Dan Vs "The Fancy Restaurant", Chris and Elise both assume the other is trying to remind him/her of their wedding anniversary. Eventually they both come clean and realize that neither of them actually remembers the date of their anniversary. Chris declares he doesn't care because he doesn't need to wait for a certain day to show Elise how much he loves her.

Real Life

Many people will testify. (Although at the same time there are many instances where both parties forget...)