A packed room nodding in unison can still be wrong. Volume was never evidence.

Agreement spreads faster than checking does. In a crowd, most heads are nodding at the nodding rather than at the claim, because it takes seconds to join a consensus and hours to examine one. That is how a whole room ends up confidently holding a position nobody in it has tested.

When you notice strong agreement, ask what would have to be true for it to be wrong, and see whether anyone can answer. Find the one person who looks unconvinced and ask them directly. Holding a private doubt for a week costs you nothing and occasionally saves a lot.