As others have said, this is what is meant by the "No True Scotsman" fallacy. For common fallacies like this one, RationalWiki usually has a very good analysis.
How will we know, when the morning comes, we are still human? - 2D
Don't worry, my friend. If this be the end, then so shall it be.
Don't worry, my friend. If this be the end, then so shall it be.