In a Spider-Man comic, Eddie "Venom" Brock's ex-wife (who is surrounded by police) addresses him as Edward over the phone. Displaying his typical lack of subtlety, Eddie immediately shouts "WHO'S WITH YOU?!"
An early Fantastic Four story has Doctor Doom switching bodies with Reed Richards. It doesn't take long for the rest of the FF to find something off about "Reed" being arrogant and stand-offish. Reed attacks in Doom's body but Johnny and Ben have figured something is up. Johnny uses his flame to create the image of a stick of dynamite about to explode. Without hesitation, "Doom" throws himself onto dynamite to save the others while "Reed" instantly tries to escape. That's enough for the team to realize what's going on and capture Doom, causing him to switch back with Reed.
In one episode of Flash Gordon, Dale distinguishes the real Flash from an impostor when the fake announces that he loves her. In a bit of a twist, Flash (the real one) is visibly troubled by this.
When captured by terrorists and forced to call Modesty, Willie addresses her by name instead of calling her "Princess". She immediately packs her bags and comes to his rescue.
Modesty & Willie also repeatedly use a pre-arranged distress code throughout the series. If either of them drops the name "Jacqueline" into a communication, that's the cue to hit the panic button.
They also use "Bertha" as code for "I need a distraction".
Captured by the Mekon and forced to broadcast a message to Earth, Digby claims he's having such a nice time it reminds him of a holiday he once had. His aunt realises he's referring to an occasion when he was wrongfully arrested and that he and Dan Dare are being held against their will.
In a Mickey Mouse story a famous opera singer suddenly not only abandons his contract but signs on for a rival — and refuses to see anyone. His former studio, not being idiots, finds this suspicious, but have no proof. When he releases a "greatest hits" collection, however, one word in each of the famous arias (all spoofs of real ones) is wrong — and the wrong words together spell out the location where he is held against his will. This trope truly enters when his ex-manager points out he would never miss a word like that.
In Frank Miller's first issue of Daredevil, Elektra forces a Mook to phone his boss and set up a meeting (so she could capture him and collect his bounty). The mook suggests a time an hour later than previously planned, which was their codeword for trouble. His boss knew he was going to be walking into a trap.
In one issue of Doom Patrol, Larry figured out that the putty-like Madame Rouge was impersonating the Chief when "he" called Rita Elasti-Girl — "the Chief would NEVER call Rita by that freak name!"
Batgirl (Barbara Gordon) and Blue Beetle (Ted Kord) know each other well. A robot impersonates his voice, yet "Babs, fortunately, is no fool, and knows Ted far too well to fall for that shit."
During the "Operation: Zero Tolerance" X-Men arc, Bastion is psychologically torturing Jubilee by forcing her to watch (faked) images of her fellow X-Men being tortured and killed. Jubilee seems about to break down until she hears "Wolverine" begging for mercy. At this point, Jubilee snaps out of it and laughs in Bastion's face, because she knows that Wolverine would never beg.
A group of Skrulls disguise themselves as the Starjammers to attack the X-Men, betting the heroes will think their allies are under mind control and thus hold back. However, the Skrull impersonating Corsair gives them away when he gloats on how "these Earthers" aren't so tough.
Storm: An interesting choice of words. Considering the real Corsair was born on Earth and proud of it!
As the title character and his lover/accomplice are Masters of Disguise, this tends to happen from time to time in Diabolik, as Diabolik doesn't always know everything. The funniest instance is when Diabolik replaced a Grumpy Old Man and a group of children recognized him because he wasn't grumpy enough.
Darkly averted in The Transformers: Robots in Disguise. In the season one finale it's revealed that Prowl's apparent descent into villainy was actually a result of Megatron's Loyalists mind-controlling him. However none of the protagonists had figured this out beforehand; in other words, they thought him becoming a ruthless murderer was completely in-character. Once he's rescued, the realization that none of the Autobots actually trust him utterly breaks Prowl's heart, causing his Sanity Slippage to kick into high gear.
Invoked in Clean Room and its version of demonic possession. One demon uses singsong repeated consonants that give away its presence in a host to the audience, if not the characters.
In Nightwing (Rebirth) #27, Spyral have turned evil (again) and Nightwing and Huntress are facing a squad of agents led by Dick's old friend from Grayson, Tiger. Dick wants to find evidence this isn't what it looks like and jokingly says "He hasn't called me an idiot yet, are we sure it's really him?" At the end of the issue Nightwing finds Tiger standing over another, badly beaten Tiger, who manages to mutter "you ... idiot ..."
Teen Titans Go!: In Issue #42, after the Teen Titans think they got all parts of Raven's personality back, Robin finds it odd "she didn't so much as frown at Beast Boy" for causing that mess in the first place. The Trigon-eyed Raven is still at large and shows a "the end?" sign.
The Punisher MAX: In the final arc, Elektra calls the Kingpin to let him know she's killed Frank. Fisk, who knows she'd be a lot more elated at having killed the Punisher, instantly deduces Frank has a gun to her head and tells her to put him on.
Ghostbusters: In the third issue of the "Mass Hysteria" arc, Peter takes a phone call for help while discussing the current problem with Walter Peck and immediately leaves via motorcycle without a word, much to Peck's surprise. Considering the caller was Dana Barrett, it's understandable.
Walter Peck: Ms. Melnitz, after taking that phone call, Peter Venkman ran out of here without a single smart-assed remark. That kind of character inconsistency is, in my experience, the reddest of red flags.
Played with in Superior Spider-Man when Doctor Octopus takes over Peter Parker's body. A complaint of fans is how Spider-Man is suddenly speaking in a scholarly and arrogant manner without his usual wisecracks. He's also far more brutal in the field to the point of killing someone. Yet, all the Avengers and other heroes do is shrug that the man is just being rougher, not seeing his complete personality overhaul as something too suspicious, testing things such as his DNA or his physiology and assuming that his different attitude is just him dealing with his own displaced guilt/grief over the death of 'Doctor Octopus'.
Lampshaded when, after getting his body back, Peter briefly acts like Octopus then says "wow, I spent months talking like that and no one noticed?"