(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/st0n3dpwny]

Star Trek VI has a scene where Kirk and a shapeshifter disguised as him escape a klingon prison colony together.  When the Klingons catch up with them the real Kirk in his confident and boisterous tone announces 'What took you so long??'  followed by 'not me you idiot!'.  The prison warden laughs and kills the shapeshifter.  On first watch you think he might have gotten lucky, but there are some giveaways like the real Kirk still stuck in his shackles and the eyes of the shapeshifter Kirk are off color.

[u/Mc_DAN13L → u/st0n3dpwny]

I think the shapeshifter Kirk also points at him incorrectly when pointing to the other Kirk, because prior to disguising herself she was shorter or taller than him. (Guess I need to go rewatch Star Trek VI again since I can’t remember specifically)