No True Scotsman

Probably one of the most misunderstood and incorrectly used of the fallacies - here's how it works:

Person A: "No Scotsman puts sugar on his porridge."

Person B: "But my uncle Angus is a Scotsman and he puts sugar on his porridge."

Person A: "But no true Scotsman puts sugar on his porridge."

Person A has committed the fallacy.

What makes someone a true Scotsman has nothing to do with their porridge sugaring habits but rather that they were born and raised in Scotland. 

If the qualification for something/someone to be in a category is not met then it is right to say that it/they does/do not truly belong in that category. The Fallacy has NOT been committed.

For example: 

Person A: "No true Christian would be involved with the inquisition"

Person B: "But all the inquisitors were part of the Roman Catholic Church - they were Christians"

Person A: "But no true Christian would kill and torture others - this is not what Christ or scripture teaches"

Person A has NOT committed the fallacy because the category of "true Christian" is defined in scripture.