There are a number of features of 'groupthink' :
The illusion of invulnerability. The fact that this decision was made by all these people suggests that it carries their collective weight and strength. There is strength in numbers and the group feels invincible.
The illusion of morality. There is something about a group that lends morality to whatever decision it makes. It's as if the decision has been vetted by each person in the room and this must be irrefutable.
Outsiders who oppose the group or its decisions tend to be stereotyped as weak, stupid or worse.
In the excitement, those who are quiet or opposed to the position are overlooked. As the majority opinion begins to take shape, the group assumes it is supported by everyone and an illusion of unanimity is formed.
A reason is put forward in favour of the decision, which often turns into a thoughtless parroting of the party line, enabling group members to ignore any evidence or arguments to the contrary.
Pressure is put on any dissenters to abandon their criticism and conform.
Blindguards are set up. Certain members try to silence dissenters and protect the leader from adverse information.
Fortunately, there are ways to prevent these problems.
become the bomb.