Untrue; erroneous
I believe the answer is:
fallacious
I believe this is a double definition.
'untrue' is the first definition.
(thesaurus)
'erroneous' is the second definition.
(thesaurus)
(Other definitions for fallacious that I've seen before include "Like a mistaken belief" , "Deceptive, misleading" , "Incorrect" , "Flawed" .)