Masked man fallacy

One example of this fallacy is Superman. Lois Lane believes that Superman can fly. She also believes that Clark Kent cannot fly. Therefore, Superman is not Clark Kent. In both premises Lois Lane is stating her belief and it may be correct but just because she believe it does not not make the conclusion true.

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"