This one will receive Rolling Updates
A character arrives at a security checkpoint where he have to verify his identity in order to gain entrance. He takes out his ID-card, runs it trough the scanner and is surprised to find that it is being denied. The receptionist then tells him that he is already checked in. This can only mean one thing: An impostor is on the loose!
If the hero is the impostor, it means that his cover is blown, and he must prepare to fight the local security team. If the villain is the impostor, chances are that he have already fulfilled his objectives and destroyed the evidence/killed the witness/sabotaged the equipment etc.
A variation of this is when the security check is especially high-tech and demands a finger/hand/iris scan. In these cases, expect cloning or time travel to be involved.
- The 6th Day were Adam Gibson uses the finger from a dead clone to gain entrance. He is discovered when a new clone with the same fingerprint tries to check in.
- Multiple people in The Fifth Element tries to check in with Korben Dallas's identity. It becomes something of a Running Gag.
- Subverted in Sneakers. Werner Brandes (who has a genius-level 180 I.Q.) knows that someone has stolen his ID card with the intent of breaking into the place where he works, and there's a (repeatedly shown) paper printout of who has entered and left the building. However, when he gets there he doesn't ask the security guards to check the entry/exit register and see if "he" is listed as still being inside the building, which would prove he's right about the break-in.
- Ultraviolet. After Violet pretends to be a courier in order to sneak into the Arch-Ministry building, the real courier shows up and her ID is rejected, thus alerting the guards.
- Sort of happens in Last Action Hero. The fictional villain "The Ripper" shows up to a movie premiere (of a later movie in the same series) and is briefly interviewed by a reporter, who thinks it's Tom Noonan (the real-life actor) dressed up as the villain. Then, the real Tom Noonan appears, dressed as himself, confusing the heck out of the reporter.
- Used in Supernatural - the brothers check into an evidence warehouse using Homeland Security IDs, only to be forced to flee once some very serious suits from Homeland Security arrive at the desk.
- A comedy variant: On Mad About You Jaimie was going back to school, but Paul had forgotten to send in her registration forms. So he, Ira, and Jaimie's friend Fran stealth sign her up for all the classes she wanted on the last day of registration, getting signatures from the teachers of the classes in question, and all had to get photo I Ds taken claiming they were Jaimie Buckman.
- A variant is used in The Legend Of Zelda Majora's Mask. Link is shocked to discover that a room at the Stock Pot Inn has already been reserved for him. Turns out there's a Goron also named Link who made the reservation. You just happened to arrive before he did. Steal his room at your peril, 'cause you can find him outside shivering in the cold later if do. Remember that this is the festival season, and all the hotels were booked solid a long time ago.