The masked man fallacy is where two true and logical conclusions are combined and lead to a false conclusion.
Masked Man Fallacy Examples
“I don’t know who the arsonist is but I know who my brother is and therefore he cannot be the arsonist.”
There are two statements that could be true, not knowing who the arsonist is a true statement and knowing the brother is true. However when we combine the two statements we possibly get a false conclusion. In this case the brother actually is the arsonist, the arguer does not know this.