The “No True Scotsman” fallacy is an appeal to purity, wherein a person dismisses criticisms of a claim because the arguments are said not to apply to the ‘true’ example.
Angus declares that Scotsmen do not put sugar on their porridge, to which Lachlan points out that he is a Scotsman and puts sugar on his porridge. Furious, Angus yells that no true Scotsman sugars his porridge.