Cover Identity Anomaly

"Homer: Hello, my name is Mr. Burns. I believe you have a letter for me.

Postal Worker: Okay. What's your first name, Mr. Burns?

Homer: (Beat)...I don't know!"

- The Simpsons

This trope is when a character, in disguise or otherwise pretending to be someone else, doesn't know some basic piece of information about who they're pretending to be. This could be because they didn't research the identity enough, or maybe the disguise and cover identity were done in a rush and they didn't think all the way through.

A quick thinking character might be able to bluff their way out of the situation, either through a Line of Sight Name, using their Real Name As an Alias, or simply pulling the Bavarian Fire Drill. This can backfire if they say the wrong thing, mess up the Trust Password, or if the person they're trying to fool is already suspicious. Otherwise, Hilarity Ensues as the character either tries to play it straight (i.e. they've somehow forgotten their first name, birthdate, hometown, etc...) or just tries to awkwardly escape. In drama, this might lead to a tense situation where the character is found out and has to escape without being caught.

Compare Stereo Fibbing, for when two characters have to make up a lie on the spot to get out of this situation.

Animated Films

 * Averted in Toy Story 3. Barbie uses an astronaut suit to disguise herself as Ken to talk with the Bookworm. The suit covers her head and face, and almost her whole body...except her feet, and she forgot to change her high heels, which the Bookworm sees as she walks away. Fortunately, the Bookworm dismisses the high heels as one of Ken's idiosyncrasies.
 * The title character of Mulan does this when asked her name. Her family name (which she must use as part of the cover story) is Fa, but she hasn't thought of a personal name, leading to a very awkward conversation where Mushu tries to secretly help her come up with a name.
 * One of his suggestions is Ah Chu.

Comic Books

 * In the early 1990s Spider Man arc where Peter Parker's parents returned from the dead, May realized they were imposters when they refer to the wrong date for their anniversary, indicating that they somehow didn't know about their secret wedding several months prior.

Film
"Han: [sounding official] Uh, everything's under control. Situation normal.
 * Played with in the movie Superbad, where one of the teenage kids gets a terrible fake ID that only has the name "McLovin".
 * In Romy and Micheles High School Reunion they are pretending to be "successful businesswomen" to impress people. It is only when a waitress asks them what business they are in that they realise they may have neglected a vital element of their cover story.
 * During a dream sequence, Michele actually supports her cover by rattling off a complex but plausible method of developing the adhesive. Interestingly enough, it wasn't scripted - the actress improvised thanks to actually having an education in science.
 * In Star Wars a New Hope, Han and Luke are disguised as stormtroopers aboard the Death Star, to rescue Princess Leia, when they're forced into a shootout with some real stormtroopers. After killing them, Han rushes to a com unit to try and fool the troopers on the other line that nothing's wrong, and the following conversation takes place:

Voice: What happened?

Han: Uh, we had a slight weapons malfunction, but uh... everything's perfectly all right now. We're fine. We're all fine here now, thank you. How are you?

Voice: We're sending a squad up.

Han: Uh, uh... negative, negative. We had a reactor leak here now. Give us a few minutes to lock it down. Large leak, very dangerous.

Voice: Who is this? What's your operating number?

Han: Uh...

[Han shoots the intercom]

Han: [muttering] Boring conversation, anyway. LUKE! WE'RE GONNA HAVE COMPANY!"

Literature

 * Skulduggery Pleasant. Scapegrace has a problem that causes him to invent a twin brother, of course he gets every detail right and thinks of everything....except one of the names.
 * In Harry Potter, Harry & Ron use Polyjuice Potion to pretend to be Crabbe & Goyle, but their infiltration of Slytherin House is stymied by the fact that they don't know how to get in to Slytherin's chambers. They ask a passing student, but she's from Ravenclaw. Fortunately, Crabbe and Goyle are so dim that they're not really acting out of character.
 * They asked the Ravenclaw where the common room was, and were lead there by Draco. They had an Oh Crap moment when Draco asked them the password, but were saved when Draco remembered it before they did. Other instances of them being out of character for Crabbe and Goyle are when Harry didn't know what Azkaban was and when both of them forgot to laugh at a Daily Prophet article where Ron's dad gets in trouble with the Ministry.
 * In the movie, Harry almost gives it away by still wearing his glasses. He quickly excuses them as reading glasses, causing Malfoy to remark that he didn't know "Goyle" knew how to read.
 * The specific example of the location of the Slytherin common room comes up again in Deathly Hallows, when Scabior says people have claimed to be Slytherins when caught by his gang of Snatchers but couldn't say where the common room is.
 * Alex Rider very nearly gets himself killed because of these.

Live Action TV

 * The British comedy series Spaced features Tim and Daisy, who are pretending to be a couple in order to rent an apartment. Though they attempt to be as thorough as possible by learning a multitude of mundane details about each other ("I forgot what you got for your fifth birthday!" "Miniature drum kit"), they get caught in their lie while fumbling around regarding what day they had sex first vs. what day they kissed first.
 * A much worse mistake is barely averted as, just before they go in, they realize they've never learned each other's names.

Real Life

 * This is the point of "Security Questions" on websites when you need to reset your password; they're intended to foil someone pretending to be you, by asking questions that only you should know the answer to.
 * Some places invert this and actually advise you to put something that is completely wrong(but that nobody would guess) as the answer, for example if the question is "What is your birthday?" put "Puppy", so that if the pretender does find out your info they can't get into your accounts.
 * Standard tradecraft for spies when communicating is, to give a hint to their controllers if someone was faking their communications, to have a subtle challenge code that, assuming the spy hadn't gone full Double Agent, would be something the faker wouldn't be able to respond correctly to or even realize they were expected to respond to. The same principle was used for dead-drops: often a legitimate drop required some subtle thing at another location to verify the drop.
 * One quick check used in stores to challenge customers who appear too young for the purchase (typically alcohol and/or cigarettes) and might have a fake ID is to ask what year they were born. Someone faking their age is likely to trip up and give their actual birth year or obviously have to stop and think about what year they would have had to have been born to be legal.

Video Games

 * One mission in Mech Assault has the player pilot a captured enemy Thor into a Word of Blake base to download some intel from their computers. The disguise almost works, until one of the Blake officers asks you to transmit your ID code or be fired upon. Fortunately, they wait just long enough for Foster to download the intel.
 * In Phoenix Wright Ace Attorney: Trials and Tribulations, an impersonator doesn't know that the person he's imitating recently suffered an injury that made him . When a witness who was fooled by the imitation testifies that the person was , Phoenix has to point out that it makes no sense.
 * In Tales of Symphonia Dawn of the New World: .   lacks a lot of information that would be expected from a resident of  Subverted in that
 * In a mission in Hitman 2: Silent Assassin, Agent 47 can adopt a disguise of a "Lord Sinclair" to get close to his target (a female doctor). If she asks 47 for the name of "his" wife, though, he won't know what to say before eventually randomly coming up with "Elsie" (which is way off the mark), blowing his cover.
 * In Saints Row the Third, the Boss disguises himself as Cyrus Temple, the leader of S.T.A.G., to infiltrate their base and save Shaundi. While Magic Plastic Surgery means the looks and voice are perfect, the Boss makes little to no attempt to try and speak like Cyrus. Depending on your chosen voice and gender, this can result in "Cyrus" doing things like hitting on his subordinate, talking about how cute a guy is, or speaking in various accents.

Western Animation

 * The Simpsons:
 * The page-quote above, where Homer attempts to pretend he's Mr. Burns...except he doesn't know what Mr. Burns' first name is.
 * When Homer pretended to be the pianist from the movie Shine. When asked for his name, he responds "Shiny McShine".