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.
Form
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"