

The best existing phrase I have is, “Dropping your guard.”
It doesn’t necessarily imply anything about why you dropped your guard, and only slightly implies that the bad thing happened while your guard was down.
We could coin a new phrase here by brainstorming. I’ll start:
Fumbled the shield





That’s not how that works. A smart person would recognize that there are degrees of smartness and degrees of immunity. Even if we accept that smarter people are less susceptible to propaganda, there is no threshold of smartness that grants immunity to propaganda.
The most important service one can provide to a kind-of-smart person is to tell them when they’ve said something stupid. I’m happy to provide this service. Please rate 5 stars.