Based on my past psychology classes, and my own interpretation, the layman can point to a classic case of GT quite simply Take a decision made by a group and analyze its sensibility at a very low level.
Members know and like each other and want to preserve group harmony.
There are perceived to be penalties to members of the group who disagree with the majority position. The complexity and importance of the decision when combined with a tight deadline put group members under great pressure. Groups suffering from groupthink were often found to be making decisions in secret limiting the extent to which outsiders can be included into the group decision making process. Illusion of Invulnerability. Overconfidence often caused from applying the lessons of past successes to new situations in which the underlying assumptions of the past decisions no longer apply.