The No True Scotsman Fallacy is essentially this:
A statement is made like such- "True Scotsman don't put sugar on their corn flakes."
When presented with an instance of a Scotsman that puts sugar on their corn flakes, the rebuttal is that the man surely can't be a true Scotsman, evidenced by the fact that he just put sugar on this corn flakes.
The statement "Scotsman don't put sugar on their corn flakes" is taken as a definitional statement of what constitutes a 'true Scotsman.'
So, I was an atheist and a skeptic for a time, but I have been confronted by atheists that say that I must not have been an atheist and a skeptic, apparently because anyone who was a 'true atheist and/or skeptic' would never have abandoned that position.
That is the No True Scotsman Fallacy. As I said, it takes a sharp eye to see it, so perhaps we ought not be surprised my dear friend Harry missed it.
heheheheheheh