Splendid!! Your disguise is so flawless that you are even more beautiful than the average woman, so much so that you become a Chick Magnet (to girls AND boys), and so refined in manner they would never suspect you were ever a man, right?
Unfortunately, being a Hot-Blooded young male all your life has habitually hard-wired your vocabulary to use the masculine pronoun of ore to refer to yourself, something that an everyday girl (who uses watashi, the polite gender-neutral pronoun) or even an upbeat and immature Genki Girl (who uses atashi, the informal-feminine pronoun, or if extreme boku, a boyish one) would never be caught dead doing.
Cue a hurried and panicked attempt to correct yourself mid-sentence, which thanks to the Rule of Funny, almost always keeps the cover from being blown.
Note this is not so much a problem with female characters disguising themselves as boys, as it is not grammatically incorrect for a male to refer to himself as watashi, though amongst his peers he would be considered unmasculine or excessively polite for doing so. However, if she tended to use atashi all the time, her cover is very likely to be blown.
A grammatical plot complication most commonly found in Japanese anime gender bending and Twelfth Night Adventure comedies, and occasionally works produced in countries that have gender-specific pronouns or adjectives in their languages (as well as verbs that betray the speaker's gender in first person singular).
If you're speaking about another person, that's Pronoun Trouble.
- In Ah! My Goddess, When Keiichi was temporarily caught in a Gender Bender by Skuld's pudding, he tried to play himself off as being somebody else when his sister and friends came around looking for him (for the sake of protecting The Masquerade of the presence of goddesses in his home). He was doing well until he referred to himself with the masculine ore pronoun, rousing suspicion in his guests.
- Invoked in Ayakashi Triangle: Haya first meets Matsuri while pretending to be a gender-bent version of her own brother. Once she has the information she came for, Haya deliberately breaks her cover by ending her sentence with -tte kanji (a feminine mannerism particularly associated with Gyaru Girls). The English translation renders this as a Valley Girlish use of Like Is, Like, a Comma.
- Boys Run the Riot: When Ryo's classmates start talking about their favorite characters in a drama, he pipes up to agree with Kashiwabara's choice, but he accidentally uses the masculine ore, hastily switching to uchi (and a male character) when he sees the faces of the others. The English translation adds an extra line to get across the same general idea:
Ryo: Uh, I-I mean, if I had to choose a girl, like me, it'd be Hana...
- In Demon Slayer: Kimetsu no Yaiba, in the Red-Light District arc, disguised as a girl, Zenitsu was knocked out by Daki while infiltrating the Red Light District. The two grateful little girls he protected from her wrath were confused by his reflexive use of the masculine-prefix of ore to refer to himself when he murmured "What just happened to me" as he awakened from said concussion. He barely prevents his cover from being blown by the excuse of "I had a strange dream that I was a man" with the feminine-prefix of watashi... Only to blow said cover wide open again when he when he reacts to their adorable blushing gratitude with a flirtingly delivered "It's a man's duty to protect little girls."
- In Girl Got Game, Sweet Polly Oliver Kyo accidentally uses atashi once early on, but immediately corrects herself and uses boku consistently from then on. Later, Yura says that she should be using atashi since she's a girl... then claims he was joking.
- Happens to Psy in an episode of Heroman, when Holly forces him and Joey into crossdressing as a disguise. Though, considering these are American teenagers in the middle of California that we're talking about here, there's probably some kind of Translation Convention at work.
- Amawa Hibiki of I My Me! Strawberry Eggs almost accidentally breaks his cover on the first day of his teaching job, in front of the whole school through this trope. Rather clunkily translated in the dub with "As a man... uh I mean as a woman."
- Natsuru Senou of Kämpfer falls into this trope so many times it's a wonder Kaede Sakura, the only girl who does not know his (identically-named, at that) female persona and him are one and the same, didn't catch on. She does catch on eventually.
- Discussed in Love Me For Who I Am, when Mei, a transgender girl who's still struggling to come out of the closet, switches from boku to watashi.
- Shiratori Ryuushi, Dogged Nice Guy and The Chew Toy of Mahoraba, was dressed up (against his will and while asleep) to look like a very cute and beautiful blonde. When confronted with Chiyuri, one of his landlady's five personalities who does not know him, he accidentally uses boku (polite masculine pronoun) to refer to himself. Subverted in that Chiyuri finds this contrast quite Moe, and that although unusual for a girl, boku is used by the nicer-tomboys out there as a pronoun.
- Kuranosuke from Princess Jellyfish is a habitual crossdresser who generally has no problem passing as female. Still, very often he slips into using ore, much to to the horror of Tsukimi, who is terrified lest her anti-male otaku friends find out his true gender. She usually covers by shouting "Ole!" and trying to make it seem as if he were talking about flamenco instead.
- Referenced to in Wandering Son. The two transgender protagonists, the female-to-male Yoshino Takasuki and the male-to-female Shuichi Nitori, had a conversation about this. Nitori still uses boku, despite being very feminine and even when dressed as a girl, and Takasuki uses the gender neutral but socially feminine (or formal) watashi. They both decide the pronouns fit them, so they'll keep on using them.
- In Your Name, when Mitsuha is inhabiting Taki's body for the first time, she accidentally uses the first-person pronoun watashi while with Taki's classmate. While not strictly a female-only pronoun, in this context (male high school students) it would come off as effeminate. She then tries to correct herself by switching between Japanese Pronouns — specifically watakushi, boku, and finally ore — much to his friends' confusion.
- The English dub instead has a bizarre exchange where she calls herself a girl, then a "gal", then finally a guy, and follows this up with "Guys just wanna have fun."
- The translations for the novelisation and manga have her try various apologies ("'Scuse me", "Pardon me", and "Sorry") before hitting "Whatever", which his friends accept.
- Real world, non-Japan-related example: in Dances with Wolves, they had a consultant for the Sioux language used in the film. Unfortunately, she was female, and apparently they didn't bother to specify which speakers were male. Sioux men talk very differently from women — apparently, the movie comes off to people who speak Sioux as if it'd had Viking berserkers talking like Monty Python housewives.
- The first known instance of this trope is in a Lord Peter Wimsey short story of the 1930s. Lord Peter watches a sexy French girl berating a man. In French, naturally. He's smart enough to figure out that she is really a man in drag who turns out to be a notorious jewel thief. The story is titled "The Article in Question", although the reader does not realize until the end of the story just how accurate a title it is.
- Doctor Who: The Thirteenth Doctor, as the Doctor's first female incarnation, does this a few times as part of a Running Gag. For instance, she calls herself the "Marvellous Apparating Man" when appearing in the Alexandria Gallery in 1834 before quickly correcting to "Apparating Lady", and she doesn't realize at first that she's the one being addressed when a police officer calls her "Ma'am."
- In Breath of Fire II, one possible tenant for your town is a guard who stutters when he says ore. He's not only a woman, as it turns out, but the princess of Highland in disguise.