This is where a claim is made about a group, what they are all like. Then if there is a contrary example, then the criteria of group membership is shifted rather than reasoning the argument logically. Words like ‘pure’, ‘real’ or ‘authentic’ are banded about. The name of the fallacy really sums it up with the well-known example:
All Scotsmen like porridge for breakfast.
But what about Angus over there, who eats eggs.
Then he’s no true Scotsman.
The most common form of this I see is about being a ‘real’ man. It’s never men defining themselves. It’s women trying to elicit some kind of behaviour (such as paying for everything) because that’s what ‘men’ do. Come on, why don’t you be a man about it (and do what I want you to). Just “man up!” now (meaning deny your feelings and subjugate your needs).
I’ve sometimes seen petty squabbling on forums based around this fallacy, usually around identity or group membership. People on Forever Alone forums are goaded because they are not “Forever Alone” enough. No real FA would have dated in the last year, even if it led to nothing or a person was wholly stood up!