A logical fallacy combining an ad hoc definition with an equivocation. One version follows:
Person A: No Christian would commit murder.
Person B: What about Reverend Paul Hill?
Person A: Paul Hill wasn't a real Christian.

Person A has committed a No True Scotsman fallacy. The second statement can be reduced to "No Christian who wouldn't commit murder would commit murder".
The original version, quoted from http://www.infidels.org/news/atheism/intro.html :

”Suppose I assert that no Scotsman puts sugar on his porridge. You counter this by pointing out that your friend Angus likes sugar with his porridge. I then say "Ah, yes, but no true Scotsman puts sugar on his porridge.”

This is an example of an ad hoc change being used to shore up an assertion, combined with an attempt to shift the meaning of the words used original assertion; you might call it a combination of fallacies.

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