Amnesiac Liar

For reasons undisclosed, Alice has told a falsehood to Bob (such as her name being Claire). Alas, an unfortunate accident later, Alice gets Easy Amnesia! (Un)Fortunately Bob is present to tell her all she needs to know.

Expect disastrous results, one way or the other.

May also be used by a Chessmaster to conceal their Amnesiac Dissonance more fully, and make their lie convincing - because now, everyone involved has no reason to disbelieve it.

Compare Criminal Amnesiac, where Bob deliberately lies to Alice about who she is to get her on his side.

Examples of Amnesiac Liar include:

Anime and Manga

In Death Note, both Light and Misa lose their memories as a result of Light's Memory Gambit. Unfortunately, this also leaves Light with no idea as to why he's Misa's boyfriend, leading to severe awkwardness when L quizzes them on their feelings for each other. She is super hot he is a teenager. No extra explanation is needed.

Film

In Santa with Muscles, Hulk Hogan pretends he's Santa Claus and gets amnesia while in the suit.
In The Long Kiss Goodnight, Samantha is really a secret agent/spy. Because of her job, she had to lie about being together with a guy who was actually a mark. She almost gets killed, instead being caught and tortured (and her old mentor gets killed because of this).

Literature

Sherlock Holmes manages to do it to himself in Michael Kurland's Professor Moriarty novel The Empress of India. He has a secret identity as a criminal, as a way of keeping an eye on the criminal underworld. When he suffers a Tap on the Head and wakes up in this lair, he deduces that this is his true identity, and proceeds to become a successful criminal.

Live Action TV

Happens in at least one Mission Impossible episode. In the new series, Shannon almost ends up marrying the villain's son.
A minor example in Smallville: During the period that Clark didn't know Chloe knew he had powers, he developed amnesia. Chloe talked him through his powers and told him he used them for good, but since she didn't know his origin, she assumed he was a meteor freak.
Samantha Who practically has this trope as part of its premise.

Web Comics

The webcomic Picatrix has an example with a guy who believes himself on honeymoon with the heroine. Naturally, this doesn't end well for him when he makes advances on his "wife"...
- Then it's inverted, and more hilarity ensues.

Western Animation

"The Forgotten", one of the early, unusual episodes of Batman the Animated Series, had this happen to an undercover Bruce Wayne disguised as a homeless guy. He gets kidnapped by a greasy, overfed slavedriver who runs a mining operation, while Alfred is trying to find him. Very poignant episode, as Bruce gradually uncovers his memories and retakes his levels in badass.
Another Batman example happens in the Batman the Brave And The Bold episode "The Mask of Matches Malone": a hit on the head causes the hero to believe himself to be Malone, his undercover gangster identity.