The Masked Man Fallacy is when an intensional statement is confused with an extensional statement, and a false conclusion is reached.
Intensional – Something that MUST be true about something.
Extensional – Something that IS POSSIBLE to be true about something.
I know, super confusing, a total brain melter. I apologize. Here is a soothing example.
The kid with the baseball cap ‘s name is Sammy.
The homeowner thinks the kid with the baseball cap egged his house last night.
Therefore, the homeowner thinks Sammy egged his house last night.
In the conclusion ‘kid with the baseball cap’ was substituted with ‘Sammy’, this is wrong because in these statements the homeowner doesn’t necessarily (intentionally) know the kid’s name. So he thinks nothing of Sammy, only the sweet revenge he’s planning for that little turd in the baseball cap.