I was talking recently to a fellow liberal who happened to be a Hindu making correctly scathing attacks on UKIP, media bias and misrepresentation and, you guessed it, Islam. Again I was somewhat frustrated that an intelligent and informed guy was getting so much right and yet made one big error. I see it so often and have been involved in debating it here and here that I had to answer my critics here
The No True Scotsman fallacy is a well-used fallacy in debates about religion with religionists. As wiki defines:
No true Scotsman is an informal fallacy, an ad hoc attempt to retain an unreasoned assertion. When faced with a counterexample to a universal claim, rather than denying the counterexample or rejecting the original universal claim, this fallacy modifies the subject of the assertion to exclude the specific case or others like it by rhetoric, without reference to any specific objective rule.