Revealing Hug

Five minutes later, Chane discovers that her wallet is missing...oh, and her voice too.

Jazz: Keep hugging. We'll see who hugs last.
Vlad: What?

Jazz: N-Nothing.

—Secret Weapons, Danny Phantom

Two characters give each other a hug. Everything looks nice and friendly (or loving), but then Alice drops the act and allows her real feelings to show through in their expression, behind Bob's back where he can't see. But the audience can see, and so this allows them to be clued in on two things:

Alice has some sort of secret
Bob has no idea, and Alice is deliberately trying to keep it that way

The face Alice makes is important in determining just what the secret might mean, since the audience can't tell from context or anything. For example:

Act: Alice is planning to kill Bob's dog
Face: Evil (see Psychotic Smirk)
Meaning: "Hahaha, little does Bob suspect, I'm planning to kill his dog"

Act: Alice has already killed Bob's dog accidentally, and is scared of being caught
Face: Worried
Meaning: "Oh Crap, if Bob finds out I killed his dog, he'll be so mad!"

Act: Alice killed Bob's dog on purpose, but now regrets it
Face: Guilty
Meaning: "Oh god, Bob is being so nice to me, he doesn't even know I'm the one who killed his dog"

Act: Alice is acting sad that the dog is dead, and Bob comforts her
Face: Happy
Meaning: "Finally I get to hug Bob, I've waited so long to do this..."

Act: The vet tells Bob his dog is going to be okay after being hit by a car, and Bob hugs a supportive Alice in joy.
Alice's Face: Disappointed
Meaning: "Oh God, the little monster just can't die!"

Act: The vet tells Bob his dog is going to be okay after being hit by a car, and Bob and Alice are both happy, but instead of an Accidental Kiss, hug.
Alice's Face: Melting with love
Bob: "Um... you can let go now..."

Act: Bob's dog is in surgery at the vet, with its fate uncertain. Alice knows the dog is really an alien trying to end Bob's entire family, and she tried to kill the dog to protect him. The dog is intelligent and knows full well Alice knows it's not a real dog. Bob is none the wiser, and hugs Alice because he needs support.
Alice's Face: Fear/Concern
Meaning: "I may have to try again to kill the beast, and break Bob's heart in the process!"

And once the hug is complete, Alice schools her face back to normality so Bob continues not suspecting a thing; she's taken what may be her only chance to let her real feelings out. Of course, the trope works just as well if dogs aren't involved at all. (Children, for instance.)

Anime and Manga

As seen above, Huey from Baccano! shows his true colours when hugging his young daughter Chane, who just offered to become voiceless so she could keep his secrets. Not that we didn't know Huey was a bad customer anyway...
When Light embraces Misa in Death Note, his face is utterly diabolic. As his thoughts make clear, his "feelings" for her are simply the next step in using her as an Unwitting Pawn. This is an interesting example, as Misa had told him it's OK to just use her for his own advantage, regardless of his feelings, not a few moments before. Not that it would have stopped Light even if she hadn't.
In the second episode of Darker than Black, Hei has this when comforting Chiaki in his Li persona. While he speaks in a loving, idealistic manner, the audience can see the actual expression on his face is cold, as are his eyes. This is close to being an example of the "evil" veresion, as Hei is on assignment to kill Chiaki, but it's never clear whether he planned to go through with it (which is moot, as she was a Doll programed with the memories of the real Chiaki to trap him) or would have looked for a way to save her.
From Fairy Tail, Jellal (the evil version) gives one to Erza when he convinces her she broke through Zeref's control on him with The Power of Love. Naturally, he's Not Brainwashed.

Film

Oliver and Company: Jenny tells Georgette that Oliver has been kidnapped, and Georgette lays her head on Jenny's shoulder, seemingly sobbing. As Jenny consoles her, we see that Georgette is actually snickering, because she was the one who helped with the "kidnapping". It turns into an Oh Crap face when Jenny says they're going to go get him back.
A less dramatic, subtler example with the Cooldown Hug Nikolai gives Kirill in Eastern Promises. It's not as if Nikolai's oblivious to his feelings, though.
- A better example might be the hug in the basement, after Nikolai has been offered his stars. It lingers just long enough for Kirill to start looking like he half-ways expects a kiss; then Nikolai pats his cheek and moves away.
North by Northwest - Roger is on the lam, meets Eve on a train, they get on very well...as he holds her we see her cool, calculating face, having slipped a note to his pursuers in the next compartment.
Subverted in Sleeping with the Enemy, as we the audience already know that Laura doesn't want to have sex with her abusive husband. But for his benefit, she pretends to be aroused as he makes advances to her, right before we see her upset face over his shoulder.
A humorous one in Charade, when Audrey Hepburn has just learned that Cary Grant has been lying to her about who he is and why he's hanging out with her. Not knowing she knows, he gives her what's intended as a comforting hug while telling her he's moved into the room adjoining hers. The way her eyes suddenly get very wide is hilarious.
In The Godfather Part 2, during Mother Corleone's funeral, Michael and Fedo reconcile with a hug. As Mike embraces him, he makes a signal to his hitman Al to kill him.
At the very end of Oldboy, when Dae-Su hugs Mi-Do, we see his smile slowly change into a mask of grief. It is up to the viewer what this means, though it seems to imply that the hypnosis may not have worked.
In the 2001 TV movie version of Othello, Christopher Eccleston's character gets a really good one while hugging (John) Othello after the latter's promotion. That look makes it clear to anyone in the audience who wasn't already aware of the plot that Ben Jago is NOT the man Othello should be depending on.

Live Action TV

This trope gets significant usage on Heroes. HRG (Noah Bennet) has done it to members of his family more than once, and it happens with the Petrellis fairly often too. Even Sylar has done it at least once.
On V 2009: Ryan is seen over his fiance's shoulder as he hugs her. The audience knows he's troubled because Valerie just found out she's pregnant, and Ryan is a Visitor, but she doesn't know that. To complicate matters, he genuinely loves her.
Neds Declassified School Survival Guide: As part of their plan to convince Ned's Clingy Jealous Girl Missy that he and Moze are dating, Moze hugs Ned... and holds on after Missy leaves.

Ned: Um, you can... let go now.
Moze: Huh? Oh, right. I know...

Done in an episode of Smallville where Clark hugs Lana. A close-up of his face reveals a distortion and an evil smile revealing him as Bizarro to the audience.
Used in Xena: Warrior Princess, in the episode "The Rheingold". Xena "confesses" her love for Odin while weeping, he embraces her, and we see her shifty eyes and suspiciously calm expression as she continues to make weepy sounds.
In Friends, Rachel stops short of confessing her feelings to Ross when she sees him and Emily (his bride-to-be) kissing and instead wishes him a great marriage with a "congratulations" hug, while looking utterly miserable when she hugs him.
Lost: Sun tells Jin, "I have never been with another man," and hugs him. As soon as she's in the hug, her face changes, and we later learn that this was a lie.
In the Buffy the Vampire Slayer episode, "Doppelgangland", Willow hugs her vampire counterpart before sending her back to her own reality. As they hug, VampWillow grins mischievously. Two seconds later, Willow jumps away:

Willow: Hands! Hands!

- In "The Dark Age", Jenny is temporarily knocked out when a demon-possessed man breaks out of his cell. As a guilt-stricken Giles later holds her, her eyes flash in an "uh-oh" moment.
- "Becoming, Part 2". Buffy realises that Angel has got his soul back and hugs him. As Buffy does so her eyes widen in horror because she can see a vortex to the hell dimension opening behind Angel; she tells Angel to close his eyes and does what she must.
Being Human (UK) had Owen hugging his fiancee, trying to reassure her that their house isn't haunted. He is, of course, staring straight at Annie, complete with Psychotic Smirk, while he tells the woman he's hugging he "can't see anything".
- The U.S. Remake has Danny hugging his girlfriend after convincing her that his ex-fiancee Sally's death was accidental. In actuality, Danny killed her intentionally.
Drake on Drake and Josh hugs his Girl of the Week while making the "Call me" gesture to another girl.
Pick a Soap Opera. Any soap opera.
- A good example is seen on One Life to Live. After a character expresses anxiety about her impending wedding (due to being widowed very soon into her first marriage), her fiance reassures her. She laughs, insists that she's fine, and they hug. Of course, we see her secretly upset face over his shoulder. . .but the trope is then played twice when we see HIS equally worried face—he is fully aware that she's only pretending to be okay.
Glee had Will's wife Terri discover that she wasn't actually pregnant with their longed-for baby. She plans to break the news to Will gently, but after he starts by giving a little speech about how important family is to him and how much he's looking forward to being a father, she lies and tells him that she's discovered she's having a boy. Her expression of utter panic over his shoulder as he hugs her in joy is tragically hilarious.
In Noah's Arc, Guy gives one to Trey, taunting Alex in the process.
Occurs in the second season of the HBO series Rome. Atea is reconciled with her son Octavian, but as they embrace, you see a look on Atea's face that implies she's just doing it for the power. Additionally, when Octavian and Mark Antony embrace when they join forces. The looks on both their faces make it clear that neither of them would be doing this if it wasn't absolutely necessary.

Professional Wrestling

Eddie Guerrero vs Chyna vs Chris Jericho for the Intercontinental championship some years ago. Jericho knocked Chyna down, Eddie did the same to Jericho, and then went to check on Chyna to make sure she was alright. In the process, he accidentally pinned her, winning the Intercontinental championship from her. After the match, he explained himself and was as shocked as she was. Chyna apparently forgave him, and they hugged in the ring...except the camera revealed an excellent smirk on Eddie's face behind Chyna's back. Seems he's not so upset about winning the championship after all.

Web Original

Oddly subverted in Ruby Quest, which would seem to be asking for this kind of thing. At the end of the first day, Ruby and Tom give each other a badly needed hug. Then Tom opens his eyes... and, well, he opens his eyes. He has to open them sometime, right?
- After reading the story to its very conclusion, and re-reading to this point, there are three probable reasons for this seemingly out-of-place dramatic moment: Perhaps he is feeling the first effects of the arsenic-laden fish. Perhaps he is vaguely aware, despite the amnesia, that he is hugging the person who once killed him. Perhaps he never had amnesia, is acutely aware that she has amnesia, and is biding his time until he gets out of his cell and the Metal Glen, and away from that maniac Red. Admittedly, his later reactions seem genuine enough to preclude that last option, but the penultimate panel remains ambiguous...
The Suspiciously Evil Sloth meme is based on this trope, adding humorous captions to a picture where a man is hugging/holding a sloth, and behind his back, the sloth seems to be making a Psychotic Smirk.

Web Comics

This Penny and Aggie page.
Near the end of Avalon, Ceilidh breaks up with Joe and goes to Phoebe for comfort. Cut to her smiling serenely, probably either out of relief that she wasn't staying in the doomed relationship, or, going by later events, relief that he was out of her way.
When Emily tells Ash in Misfile that she's agreed to go on a date with a guy she's just met, Ash says that she's happy for her. She looks like she's about to break down into tears when they hug because she wishes that she (actually a "he" deep down inside) could be the person Emily's going out with.
Subverted in this strip of Sluggy Freelance. It's not Torg's face that reveals his emotions (we don't even see his face), but rather his inability to return Zoe's hug. The subversion comes in when Zoe notices it (caught up in the moment, she could have easily not realized Torg never hugged her back).

Western Animation

A Lampshade of this occurs in Danny Phantom when Jazz adds an unnecessary commentary as she hugs her "Uncle Vlad". ("Keep hugging, we'll see who hugs last.")
Toyed with in Avatar: The Last Airbender when Aang and Iroh rescue Katara and Zuko, who had been Locked in a Room. While both pairs are genuinely happy to be reunited, Aang and Zuko aren't thrilled to be working together, and shoot each other Death Glares across the shoulders of their hug-partners.
This short animated film Red where Little Red Riding Hood refuses the advances of the wolf (who looks like a slightly feral child wearing a wolf costume) after spilling the cakes she was going to bring to her grandmother. After leaving brokenhearted, Red encounters a unicorn-horned rabbit, which she hugs (not noticing the evil toothed grin it makes before mutating into a bear-sized horned monster that was ready to devour Red before the wolf comes back in time to save the day.