In communities, a "sacred cow" is a foundational or definitional idea (c.f. DoctrinalElement?) that, though having lost its importance or value with time, cannot be abandoned because of the effort invested in it in the past. As such, the "sacred cow" can be symbolic of past success, struggle, or sacrifice. The presence of sacred cows leads to inhibited communication and GroupThink.
Unlike LandMines, which generate heated discussion when touched off, agreement upon the nature of sacred cows is usually strong enough that those who challenge them are quietly marginalized or ignored.
Since the 1960s, the DFL party (U.S.) has had a special reverence for "affirmative action" -- that is, extending preferences in hiring, government programs, college admissions, to blacks and other racial minorities that were, historically, targets of racism. Affirmative action became a definitional, foundation issue for the party during the 1960s civil rights era. Today, a generation or two later, support for affirmative action is waning, yet the party cannot change. Affirmative action has become a "sacred cow" within the party regardless of the fact that most people within the party do not, privately, support it.
Reserve the right to be smarter tomorrow than you are today; create no sacred cows. Do not enshrine social policy in foundation documents or elsewhere where it is difficult to change.
When sacred cows are present, strong leaders who choose their timing well can sometimes get rid of them. When possible, lead the sacred cows to public slaughter, thus improving communication and reducing groupthink. Often, this is not possible because a sufficient number of group members may be clinging to old illusions.
Often the only alternative is to StartAgain with a new community.