The mistake one makes when one takes the members of a certain group as representative when in fact the group has already undergone some selection process.
(Trivial) example 1: If you ask all the people at a country music concert, you will learn that 100 percent of the population like country music :-).
Example 2: People who like formalism probably are under-represented on Wiki, because those who do may not come back.
When the selection process concerns ideas/beliefs... this may lead to GroupThink and thus MentalInertia.