A critic may describe a narrator as thiqah because the narrator is generally dependable. That does not rule out an error in an individual transmission. A broad biographical grade and the accurate preservation of one particular detail are related but distinct judgments.
If a trustworthy person supplies an addition expressly contradicted by several more accomplished transmitters, investigators must ask whether this is a specific mistake. Conversely, a compatible extra detail should not be dismissed merely because only one reliable transmitter preserved it.
Two levels of assessment: general uprightness and precision; then close examination of the actual wording, chain variants, and the relevant addition.