Masked-man fallacy

The masked-man fallacy (also known as the intensional fallacy and the epistemic fallacy) is committed when one makes an illicit use of Leibniz's law in an argument. Leibniz's law states that, if one object has a certain property, while another object does not have the same property, the two objects cannot be identical.

The name of the fallacy comes from the example:

Premise 1: I know who Bob is.
Premise 2: I do not know who the masked man is
Conclusion: Therefore, Bob is not the masked man.

The premises may be true and the conclusion false if Bob is the masked man and the speaker does not know that. Thus the argument is a fallacious one.