Faḥsh al-ghalaṭ means unusually frequent or serious mistakes. Every narrator may slip on occasion; one error does not automatically disqualify a dependable memorizer. But a sustained pattern of grave mistakes undermines reliability.
The evaluation depends on comparison with preserved transmissions, differences in chain and wording, and the assessments of specialists. Errors may involve reversing names, omitting transmitters, or confusing the wording of reports.
Difference: an isolated minor lapse and pervasive serious instability are not the same. The severity and frequency of errors distinguish this cause from the generally less severe category of weak retention (sūʾ al-ḥifẓ).