Conjunction Interruption

"Dr. Evil: Sh!... Knock-knock. Scott Evil: Who's there? Dr. Evil: Sh! Scott Evil: But... Dr. Evil: Let me tell you a little story about a man named Sh! Sh! even before you start. That was a pre-emptive "sh!" Now, I have a whole bag of "sh!" with your name on it."

- Austin Powers

When a character attempts to justify himself versus a figure of power (a parent or hierarchic superior), 90% of the time he will be interrupted after "but" or "and". More generally, a sentence starting with "But Mom..." will never be completed. The interruption is of the form "No buts!" or "Not Now, Kiddo!" depending on the situation. Inevitably leads to One Side of the Story (even worse if You Know What You Did), and if the poor character finally gets a chance to justify himself, he will get a Why Didn't You Just Say So? answer (typically when he got served with "not now").

Catchphrase Interruptus applied to a Stock Phrase. Not to be confused with a concussion interruption.

Film

 * The Austin Powers example in the page quote.
 * In the film of Harry Potter, this happens to Harry Potter at his trial with Cornelius Fudge cutting off all explanation.

Literature

 * In Honor Harrington, when someone's internal monologue gets a dramatic interruption to the face, the last word in said internal monologue will inevitably be "and".
 * Mercedes Lackey uses it so often in her Heralds of Valdemar novels that the word "But" has become a cue to trigger an interruption.

Theater

 * This trope goes back at least to Henry Fielding's 1730 play Rape Upon Rape, which is allegedly the origin of the related Stock Phrase "but me no buts". The relevant quote is too long to reproduce here, but Google Books has it.

Web Comic

 * Shortpacked: Cut her mic!

Western Animation
"Bart: But mooom! Marge: ...what? Bart: Uh, that's all I got."
 * The Simpsons:

"Homer: If if's and but's were candy and nuts..." eh, how does the rest of that go?"
 * Another one: