Open Mouth, Insert Foot

A character with a big mouth says something stupid or, better yet, self-incriminating. Usually the twin trope to or immediately followed by Did I Just Say That Out Loud?.

The trope name comes from the phrase "to put your foot in your mouth" (also playfully referred to as "foot-in-mouth disease", a pun on the livestock complaint "foot-and-mouth disease").

Closely related to Ignore the Disability.

Anime and Manga

 * Tsurubami Kamome from Medaka Box actually gets kicked in the mouth twice for doing this too often. Not that it hurts him.

Fanfiction

 * Elecktrum's Narnia Fics are full of this trope. One fic revolves around it completely, and another has a chapter revolving around the similar phrase "eating crow." Her OCs and minor characters also get into the act, especially where Edmund is concerned:
 * Tumnus unintentionally insults Edmund by implying that Edmund had worked for the white witch in "The Conscience of the King".
 * In "Black Dwarfs, Blue River" an overzealous faun angrily tells Edmund regarding the persecution of families of some satyrs who fought on the witch's side that the wives and children (who did not fight for the witch) were "Sons of traitors [who] deserve what what they get." Edmund responds cooly: "so what will you call my sons?" Oops.
 * And then there's Ilano's Right in Front of Me moment in "They also Serve"
 * In the prequel to Respawn of the Dead, Heavy accidentally insults Medic a few times. But, to be fair, it's pretty clear that Medic is only insulted by being told he sounds like Greta Garbo because he has no idea he was practically quoting her.

Film
"Howard: What if it decides to melt down a busload of nuns? How'd you like to right the headline on that one? Ben: Nun Soup?"
 * In Short Circuit Howard theorizes the potential danger of having their laser-armed robot roaming free. The stereotypical Indian guy, Ben, hasn't really grasped the concept of discretion.

Live Action TV
"Lynda: I like kissing, don't you? Jeff: Oh, yeah! Especially the tongues part. I love getting all that extra tongue. You know, sometimes I eat really cold ice cream just so that my tongue goes numb and it feels like someone elses... But then we all get lonely sometimes."
 * In Firefly, Simon does it virtually every time he tries to have a conversation with Kaylee.
 * Coupling: A form of Cannot Talk to Women, by Jeff: just about every time Jeff talks to a woman, he manages to do this. For example, this exchange occurs just when things seem promising:

"Molly: How's the hip? Mrs. Hudson: Oh, it's atrocious. But thanks for asking. Molly: I've seen much worse. But then I do post-mortems."
 * In Sherlock, we have Molly.

Webcomics

 * Code Name: Hunter: Gypsy stuffs both feet, up to her knees in when called to the office by Queen Moraine. She's not in trouble. Yet.
 * Freefall: Naiomi is busy repairing the waste waster recycling plant. The toilets will work and you can get a drink.
 * The Whiteboard: Swampy, here.
 * In Tales of the Questor, when assuring a girl that people who take her for male are foolish, do not mention her hips.

Western Animation

 * The Flintstones: Fred literally did this on many occasions.