Contrary-to-Fact Hypothesis This fallacy consists in treating a hypothetical claim as if it were a statement of fact by making a claim, without sufficient evidence, about what would have happened in the past if other conditions had been present or an event that will occur in the future.
Example: Consider the following contrary-to-fact hypotheses, none of which is provided with any support: "If you had only tasted the stewed snails, you would have loved them"; "If I hadn't goofed around my first year in college, I would have been accepted at medical school"; "If I had only been there for him last night, he wouldn't have killed himself"; or "If only I had practiced a little more on my backhand, I could have won that tennis tournament."
It is not likely that any evidence could be mustered to support such hypothetical claims. Therefore, they would probably never merit our acceptance. If there were reasons to accept them, we are rarely, if ever, given those reasons, and even if we were, there is still the question of whether they could ever count as "evidence."
Attacking the Fallacy Because the formulation of imaginative constructs is a vital part of planning for the future and understanding the past, in no way would I encourage anyone to pounce on every hypothetical construct or to refrain from exercising one's own imagination. However, if you are confronted with a substantive contrary-to-fact claim that is highly questionable, I would suggest that you find some way of getting the arguer to recognize and to admit to the speculative character of the claim. Sometimes the very act of admitting that a claim is speculative will lead one to be more open to counterarguments and to take more seriously the task of supporting the claim.
One effective way of confronting an unsupported hypothetical claim might be something like this: "Well, you may be right, but I would have no way of determining that, as I am not aware of your evidence for such a claim." There is of course no "evidence" available, but the arguer will at least probably feel obligated to make some attempt to share with you the basis for his or her speculation about the claim, and that might get the discussion on a constructive track.
["Attacking Faulty Reasoning", By T. Edward Damer]