Also very important:
Group-think

I tried to capture most of this with logcal fallacies.
But because it is so common, I think it is good to look at it separately.

From the article

These are the main behaviors to watch out for:

1. Illusions of invulnerability lead members of the group to be overly optimistic and engage in risk-taking.

2. Unquestioned beliefs lead members to ignore possible moral problems and ignore the consequences of individual and group actions.

3. Rationalising prevents members from reconsidering their beliefs and causes them to ignore warning signs.

4. Stereotyping leads members of the in-group to ignore or even demonise out-group members who may oppose or challenge the group’s ideas.

5. Self-censorship causes people who might have doubts to hide their fears or misgivings.

6. “Mindguards” act as self-appointed censors to hide problematic information from the group.

7. Illusions of unanimity lead members to believe that everyone is in agreement and feels the same way.

8. Direct pressure to conform is often placed on members who pose questions, and those who question the group are often seen as disloyal or traitorous.

9. (addition) Tendency to swing from the status quo to the complete opposite.

10. (addition) They become control freaks (and lack critical thinking).