The No True Scotsman fallacy is an informal fallacy that, similar to the Ad Hoc Rescue fallacy, where a generalization is made, then the counter is that there are exceptions to the generalization, in which the arguer then claims that said exception was never part of the original generalization.(IEP, Dowden)
Example:(IEP, Dowden)
Smith: All Scotsmen are loyal and brave.
Jones: But McDougal over there is a Scotsman, and he was arrested by his commanding officer for running from the enemy.
Smith: Well, if that’s right, it just shows that McDougal wasn’t a TRUE Scotsman.