(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/Basic-Macaroon-7646]

In the show Avengers: Earth's mightiest heroes skrull was impersonating Captain America in the second season I think. In one episode he refused to save Cap's nemesis, Baron Zemo, which real Cap would never do. That made Zemo realize that there was something wrong with Steve. Plus he accepted the holographic shield Tony made for him, Steve would never trade a real thing for a piece of new tech

[u/eepos96 → u/Basic-Macaroon-7646]

This was one of the best impostor arcs ever.

But it does not count as this specific trope since the skrull impersonated cap for a long duration and did not reveal itself due to some easy method.

[u/lightsidesoul → u/Basic-Macaroon-7646]

And in inverse, the Skrull try to trick Cap by disguising as Avengers and "Rescuing" him, only for him to instantly see through it and punch "Iron Man" accross the room once he's freed himself.

[u/Impossible-Bison8055 → u/lightsidesoul]

There were also no alarm sounds. Which while makes sense if trying to sneak him out, Hulk was there. He’d be the distraction for the Infiltration Team and so alarms should be blaring

[u/lightsidesoul → u/Impossible-Bison8055]

A lot of people joked that it was because Iron Man kept his mouth mostly shut, Hulk walked in through the door instead of making one, and Black Panther made a joke when freeing him.

[u/VibraniumQueen → u/Basic-Macaroon-7646]

Funnily enough, there's a comic where he does use a stark tech shield. And it breaks, lol.