Logical Fallacies: a Study

[Lemur]

You missed one of the most common:

13. Tu quoque

Or, you too, nanny-nanny-boo-boo!

[A] Latin term that describes a kind of logical fallacy. A tu quoque argument attempts to discredit the opponent's position by asserting his failure to act consistently in accordance with that position; it attempts to show that a criticism or objection applies equally to the person making it. It is considered an ad hominem argument, since it focuses on the party itself, rather than its positions. [...]

You-too version

This form of the argument is as follows:

A makes criticism P.
A is also guilty of P.
Therefore, P is dismissed.

This is an instance of the two wrongs make a right fallacy. [...]

Inconsistency version

This form of the argument is as follows:

A makes claim P.
A has also made past claims which are inconsistent with P.
Therefore, P is false.

This is a logical fallacy because the conclusion that P is false does not follow from the premises; even if A has made past claims which are inconsistent with P, it does not necessarily prove that P is either true or false.

Example:

"You say aircraft are able to fly because of the laws of physics, but this is false because twenty years ago you also said aircraft fly because of magic."

-edit-

And how could I forget one of my all-time favorites:

14. No true Scotsman

Or, moving the goalposts.

No true Scotsman is a logical fallacy where the meaning of a term is ad hoc redefined to make a desired assertion about it true. It is a type of self-sealing argument. [...]

When faced with a counterexample to a universal claim, rather than denying the counterexample or rejecting the original universal claim, this fallacy is employed to shift the definition of the original class to tautologically exclude the specific case or others like it.

A universal claim is of the form "All x are y" or "No x are y." In the example above, the universal claim is "No Scotsmen are brutal maniacal rapists." (No S are BMR.) The counterexample is given by the Aberdonian, who, it is implied, is a brutal maniacal rapist. The response relies on a continued insistence that No Scots are brutal maniacal rapists, and to thus conclude that the brutal maniacal and rapacious Aberdonian is no true Scot. Such a conclusion requires shifting the presumed definition of "Scotsman" to exclude all brutal maniacal rapists.