Accusation of lying means that serious evidence makes a narrator untrustworthy, yet an act of fabrication in a particular prophetic report has not been definitively established.
One relevant circumstance is a report found only through that narrator, contrary to established principles and lacking credible support elsewhere. Another is a person known to lie in ordinary speech even if no deliberate lie in prophetic narration has been proved against them.
Essential distinction: suspicion supported by evidence is not the same as demonstrated fabrication. Mere rarity or a surprising isolated narration is not automatically proof of deliberate falsehood. This defect leads to the classification matrūk (abandoned).