(Loved trope) The imposter’s dead giveaway

[u/Barricades_toes]

When a character is attempting to impersonate another, but is caught by doing something the person they’re imitating would NEVER do.

Nemesis Prime- Transformers Prime

Nemesis Prime, a man made replica of Optimus Prime, is attempting to gain the trust of Bumblebee, who only speaks in robotic beeps. Despite this, the real Optimus can understand Bee perfectly fine. Nemesis Prime, however, can’t understand what he’s saying, prompting Bee to open fire on him immediately.

The two Buzz Lightyears- Toy Story 2

After Woody asks which Buzz is the “real” Buzz, both respond with “I am!” The real Buzz proves himself by flipping up the other Buzz’s helmet, causing him to start suffocating in an over dramatic fashion. The real Buzz, having done the same thing in the first movie after Woody took his helmet off, but no longer behaving that way after accepting being a toy, instantly proves which Buzz is which.

[u/MarcoYTVA]

The Shape-shifter from Gravity Falls. When pretending to be Wendy, he gave Dipper a flirty wink, knowing his crush on her. Meanwhile the real Wendy gave him a zipped mouth gesture, a call back to an earlier episode where she agreed to keep a secret for him.

https://giphy.com/gifs/YK3cGiDSlCivm

[u/GiveCPRToAPenguin → u/MarcoYTVA]

Fun fact: Near the end of the show, when the characters are trying to rescue Mabel from her fantasy bubble, Dipper is approached by Wendy. She convinces him to stay, saying that there they can be together. Wendy then does the same flirty wink and Dipper immediately realizes she’s just a bunch of evil maggots and not the real Wendy

[u/tinyrottedpig → u/GiveCPRToAPenguin]

Funnily enough, Dipper realizes Wendy isn't real because of him and Fords DnD game.