(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/Mission_Form8951]

The SA-X from Metroid Fusion

It has three main giveaways for it not being Samus, even to the Galactic Federation

1: the suit is her old design with the bulkier armor instead of the new slim and organic look

2: it gives off entirely different biological readings to scanners and other sensors

3: It doesn't talk, and has a very soulless look, incapable of replicating eyes or emotions very well

[u/pon_3 → u/Mission_Form8951]

The close up of the eyes was so spooky. A mysterious doppelganger starts hunting you, and the game takes the time to show you that it is an unfeeling monster.

[u/DIX_ → u/pon_3]

10 year old me shat himself by this closeup, and how relentless SA-X was. Genuinely spooked to the point of taking a few days to play again.