Craig Melvin wouldn’t last a second under Miranda Priestly -- not after accidentally spilling a major "The Devil Wears Prada 2" cast secret on live TV ... and you just know Jenna Bush Hager had words waiting off-camera!

The "TODAY" host casually revealed Wednesday Jenna is gonna be in the new movie ... and when the camera cut to her, you could see the instant shock all over her face.

Jenna shut it down fast, saying that info was not supposed to be out ... with Savannah Guthrie piling it on that it definitely wasn't public knowledge.

Craig tried to clean it up, insisting he thought it was already out there -- but the awkwardness didn’t end, with Savannah later joking she was "putting it back together here" after the commercial.

The film drops May 1 ... and someone please keep Craig far, far away from any more plot details.