An excellent page on groupthink and how it affects the decision-making process of groups. Irving Janis describes eight symptoms** of groupthink:
1. Illusion of invulnerability
2. Stereotyping outsiders
3. Bounded rationality and tethered assumptions
4. Belief in inherent morality
5. Self-censorship
6. Direct pressure on dissenters
7. Mindguards
8. Unanimity
Groupthink of Irving Janis describes the 1986 Challenger explosion as viewed thorugh the lens of this phenomenon. (via Jerry)
** For some reason, this page lists only seven symptoms, but there are eight. Update: the first symptom should probably read "Illusion of invulnerability", not "Illusion of vulnerability" (thx Neilalien)...not sure why that page has so many errors. A little Googling should reveal some more reliable sources.