Spiking the Camera

A specific form of Breaking the Fourth Wall accomplished merely by looking directly at the camera. Usually done by accident, by amateur actors who happen to notice a camera is pointed at them. The trouble here is that the editor didn't catch it, allowing the character to make eye contact with the audience. Typically a film or live TV trope, especially in dramas.

Intentional occurrences fall under Aside Glance or Fourth Wall Observer.

Contrast with No Fourth Wall (thus including documentaries and reality TV), where the cast is aware of the audience, which—in turn—is aware of the production crew.

Examples of Spiking the Camera include:

Film

Hot Fuzz: Timothy Dalton looks into the camera for a second in a bar scene. Instead of using a different take, the director actually put a cash register ring on it, to draw even more attention to it.
In the opening scene of A Night At The Opera there's a woman in the background who is framed precisely in the center between between Sig Ruman and Margaret Dumont, who stares into the camera the entire time. (Here starting around 5:00)
In Love Actually, Keira Knightley's character looks at the camera as the boat she's on is pulling out of the dock. Of course, it's a film taken by an in-story amateur on her wedding day, so the slip is more forgivable. The director commented on it, because the character filming is in love with Kiera, and so by looking at the camera, it's like she's looking straight at him.
Averted in Pan's Labyrinth. It is pointed out in the DVD commentary, where one of the actors looked at the camera as he was exiting the scene, that they covered it up by digitally replacing his head in the relevant frames with his head from the last frame before he looked. It's all over in less than a second, so you don't notice it unless it's pointed out.
In Surf's Up, Nani brings an injured Cody to his uncle the Geek , really Big Z in hiding, who is shocked at seeing the documentary crew following them, staring at the camera with suspicion and indignation.
Happens a few times in Dazed and Confused, a consequence of using some very fresh actors and random locals as extras.
An in-universe version occurs in Captain America: The First Avenger, as a film director specifically tells Steve not to look at the camera during filming of a Captain America propaganda film.
In the 1956 The Ten Commandments, Sephrah warns Moses of an intruder nearby; Moses tells Sephrah "Your eyes are sharp as they are beautiful". Yvonne DeCarlo responds by staring straight into the camera, away from where she's just said the danger was.
Obi-Wan appears to do this in Star Wars just after scaring the Tusken Raiders off and ensuring that Luke is alive. He turns to the camera, doffs his hood and says "Hello there!" The very next shot shows that we have been looking through R2-D2's eyes.
One of the earliest known examples comes from the 1903 film The Great Train Robbery. The final shot of the film is of the leader of the outlaw band (played by Justus D. Barnes) looking directly into the camera and firing a gun at the audience.

Live-Action TV

Parodied in a Green Acres episode where a documentarian comes to Hooterville to shoot a film. Mr. Haney opens up a film acting class and the first thing he teaches is to not look at the camera. The others interpret this as covering their eyes or turning their back to the camera.