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The "No true Scotsman" argument is an argument of the form:
Argument: "No Scotsman puts sugar on his porridge."
Reply: "But my friend Angus likes sugar with his porridge."
Rebuttal: "Ah yes, but no true Scotsman puts sugar on his porridge."
This form of argument is a fallacy if the predicate ("putting sugar on porridge") does not follow from the accepted definition of the subject ("Scotsman"), or if the definition of the subject is silently adjusted after the fact to make the rebuttal work.
Some behaviors are actually contradictory to the label; "no true vegetarian would prefer a beefsteak to a salad" is not fallacious.