I can imagine the scene in my head but I can't for the life of me figure out what it's from. Maybe it will ring a bell for someone else?
Let me set the stage: the group of protagonists is brainstorming how to overcome an obstacle, when the ditz of the group makes a suggestion that seems gloriously simple and elegant, maybe too good to be true.
Then someone says (either genuinely or sarcastically) "yeah, great, just give us the Thing That Is Necessary to Execute the Plan!"
At which point the ditz realizes they no longer have the Thing, because of reasons likely ironically linked to the initial problem.
Everyone facepalms. Ditz is slow on the uptake.
I'm SURE there are multiple examples of this, but I can't find anything...
Related to but not: Dumbass Has a Point, Booing Crowd, Comically Missing the Point, Actually a Good Idea, Simple Solution Won't Work, Didn't Think This Through, Missing Steps Plan, Delayed Reaction