MEME REFERENCE

Nobody Expects the Spanish Inquisition Monty Python

Use when an unexpected person, event, demand, or complication abruptly interrupts an otherwise ordinary situation.

What it expresses: Surprise itself is the joke: the audience could not reasonably anticipate the intervention.
Example: A routine status meeting is suddenly joined by the chief executive, and someone messages that nobody expected the inquisition.
When not to use it: The agenda clearly announced the guest weeks ago; calling the arrival unexpected would ignore shared notice.