No true Scotsman

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") is not actually contradictory to 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 because it follows from the definition of "vegetarian."