No True Scotsman

This error is a kind of ad hoc rescue of one's generalization in which the reasoner re-characterizes the situation solely in order to escape refutation of the generalization.

Example:

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.

Ad Hoc Rescue

Psychologically, it is understandable that you would try to rescue a cherished belief from trouble. When faced with conflicting data, you are likely to mention how the conflict will disappear if some new assumption is taken into account. However, if there is no good reason to accept this saving assumption other than that it works to save your cherished belief, your rescue is an ad hoc rescue.

Example:

Yolanda: If you take four of these tablets of vitamin C every day, you will never get a cold.
Juanita: I tried that last year for several months, and still got a cold.
Yolanda: Did you take the tablets every day?
Juanita: Yes.
Yolanda: Well, I'll bet you bought some bad tablets.

The burden of proof is definitely on Yolanda's shoulders to prove that Juanita's vitamin C tablets were probably "bad" -- that is, not really vitamin C. If Yolanda can't do so, her attempt to rescue her hypothesis (that vitamin C prevents colds) is simply a dogmatic refusal to face up to the possibility of being wrong.