Groupthink is a psychological phenomenon that occurs within a group of people in which the desire for harmony or conformity in the group results in an irrational or dysfunctional decision-making outcome. Group members try to minimize conflict and reach a consensus decision without critical evaluation of alternative viewpoints by actively suppressing dissenting viewpoints, and by isolating themselves from outside influences
https://en.m.wikipedia.org/wiki/Groupthink
The most commonly used example is the Bay of Pigs invasion, where no one advising Kennedy raised any possible negative consequences. That’s seen in contrast to the Cuban Missile crisis a few years later where Kennedy actively sort contrary advice.
Surely having everyone believe the same true thing isn't actually bad - nobody calls Maxwell's equations "groupthink" unless they're trying to sell you a perpetual motion machine.