Teddy: Because you're relying on them alone. You don't remember what you've discovered or how. Your notes might be unreliable.
Alice has some agenda that leads her to tell a falsehood to Bob, such as introducing herself as a person named Claire. Alas, an unfortunate accident later, Alice gets Easy Amnesia! (Un)Fortunately Bob is present to tell her all she needs to know: as far as he knows she is really Claire, and having forgotten her real identity, she actually believes it. Depending on the tone of the work, hilarity and/or drama will ensue.
Compare Amnesiac Costume Identity, where Alice gets amnesia while wearing clothes that are not her own, with similar results, and Criminal Amnesiac, where Bob deliberately lies to Alice about who she is to get her on his side.
- 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; when she lost her memory, she apparently retained enough memory to believe that her last cover identity was her real name. She almost gets killed, instead being caught and tortured (and her old mentor gets killed because of this), but this torture allows her to regain her old memories and skills.
- Memento: As quoted above, Leonard, who has anterograde amnesia, relies on a system of notes and photos, which Teddy warns him could be unreliable. Turns out Teddy was right, and among other things, Leonard tricks himself into killing Teddy even though he's not the one Leonard is looking for.
- In Trap For Cinderella, Micky is told that her friend Domenica died in the same fire that disfigured Micky. Julia claims that she conspired with Domenica to kill Micky and have Domenica's face rebuilt to look like Micky's. Ultimately, it seems Micky was the one who survived the accident, then had her face reconstructed.
- In Unknown (2006), several of the amnesiacs remember being part of the kidnapping. Jean Jacket find that the kidnappers outside recognize his voice...
- The other film named Unknown (2011) combines this with Becoming the Mask. The main character is on a honeymoon in Europe with his wife at an expensive hotel, and they take a rental car to "explore the sights" but end up in an accident. He then wakes up in a hospital, is discharged and goes back to the hotel, where his wife is with a different man who mutually claim to be married and on honeymoon, or at least as as far as the audience (and soon the main character himself) can tell. After nearly getting arrested and returning to the hospital, the doctor concludes he has a concussion and sufficient brain damage to cause memory loss. Convinced the doctor is lying, he sets out to figure out why there is a conspiracy and what the hell is actually going on. He was in the conspiracy, and was a mercenary assassin paired with a female partner and a support crew not seen on screen until later, sent to kill a scientist at the hotel whose invention threatens existing financial interests. The car accident is a Spanner in the Works that forced the rest of the team to abandon him at the hospital and carry on with a backup member, but they expected him to wake up as normal and hightail it elsewhere on his own after an extensive recovery. Needless to say, it didn't go according to plan; his fake ID and passport were forged well enough and backed up with enough false data in the system to be considered real, so when he woke up thinking he was the non-existent person he'd been pretending to be, the hospital had no reason to think otherwise until he returned confused and they did some scans (the doctor wasn't lying or in on it), at which point they mention he may have gotten his real identity confused with a fictional character or other person as a result of the head injury. After going on what started as a wild goose chase, he manages to talk to a private detective with enough experience to know he isn't lying. After the main character leaves (long story short, "detective" = former East German secret police who drank tea with a Cyanide Pill before the clean-up crew could get any useful info) he starts thinking a bit like his old self and returns to the hotel but says he thinks he knows what's going on and to keep the "couple" nearby to prevent escape. Cue security footage of main character and his former partner scoping out the hotel months ago on a nearly-identical visit (cover-story and all) followed by Action Ensues, the female partner and clean-up crew dead, and the main character leaving the city on a train with newfound Love Interest in tow but an uncertain future ahead and no other allies or friends not faked as part of his former career. Unfortunately for the example, the entire plot after the first few minutes is "Spoileriffic"; if you haven't already (don't confuse it with the other movie just above), go watch it first if you like action thrillers.
- Inverted in While You Were Sleeping: When Peter wakes up from his coma, the rest of his family tells him that he's engaged to Lucy, but Peter doesn't remember her at all. So the family concludes he must have Laser-Guided Amnesia. note Peter becomes skeptical of the amnesia explanation as he realizes that he still remembers everything else about his life—but he gives up and goes along with his family anyway.
- 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.
- In Harry Potter and the Chamber of Secrets, Gilderoy Lockheart is a fraud who fabricated his entire career, but he successfully managed to keep the truth from getting out by generous use of memory charms on the people who actually did heroic things. When he unwisely attempts to cast another memory charm with Ron's broken wand, it malfunctions and backfires on him, giving him permanent amnesia and likely some brain damage. In the hospital he came to legitimately believe that the events in his career were true, since there was no one who knew the truth to explain to him otherwise.
- 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 when a meteor freak accidentally erased Clark's entire memory rather than just the last few hours. 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, as the protagonist has trouble finding out what was actually true about her pre-amnesia life.
- On Dark Matter, One is surprised to meet Jace Corso because that's who he thought he was; the encounter leaves him with no idea who he actually is or why he pretended to be a notorious criminal or why he was on the Raza.
- In Final Fantasy VII, the protagonist Cloud Strife narrates his past to the audience and the other characters at various points throughout the game. However, it turns out that he in fact lost his own identity and was simply narrating the past of his dead friend Zack, deluding himself into believing that was his own identity.
- 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.
- "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.