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.

Examples of Conjunction Interruption include:

Film

The Austin Powers example in the page quote.
In Harry Potter and the Order of the Phoenix, 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.