What is the narrow scope of the good faith exception to the exclusionary rule?
The scope of the good faith exception to the exclusionary rule was narrowest when the exception was first created in 1984. Since then, the exception has been expanded a great deal.
In 1984, the Supreme Court created the good faith exception in the case of US v. Leon. In that case, the Court ruled that the purpose of the 4th Amendment was to deter bad conduct on the part of the police. Therefore, in a case where the police had an invalid warrant through no fault of their own, the evidence they seized could not be excluded. At that point, then, the exclusion applied to evidence seized when it was not the police's fault that their warrant was later found to be invalid.
The rule has been expanded since then. For example, in 2009 (Herring v. US) the Court held that evidence could only be excluded if the misconduct by the officers was both deliberate and "culpable." This is a broader exception.
Thus, the narrow scope of the exception existed when it was first created. The scope has since been expanded.