Masked man fallacy

Name of fallacy Masked man fallacy
Type  Deductive Argument, Formal Argument
Description The substitution of identical designators in a true statement can lead to a false one
Example Fact 1: I know who X is.
Fact 2: I do not know who Y is.
Conclusion: Therefore, X is not Y.
Treatment Not all outcomes are covered therefore the conclusion cannot stand. In the example "I do not know who Y is" you can therefore state Y could be X. Is invalidates the conclusion "Therefore, X is not Y"