- Contradicting the reliable narrators
When a narrator’s report—whether in its chain of transmission or its wording—conflicts with what the reliable transmitters relate, it can do so in several ways that necessitate classifying the report as anomalous (shādhdh). The scholars therefore place this among the forms of criticism that pertain to accuracy, for the very motive behind contradicting the trustworthy narrators is none other than inadequate precision and retention, and a failure to safeguard the narration from alteration or substitution.
- Confusion (wahm)
Criticism also arises from confusion and forgetfulness. When a narrator errs because of them and transmits a report merely under a mistaken impression, then, once this is established by indicators that point to hidden defects and other invalidating causes, the hadith is deemed defective (muʿallal).
The subtlety and intricacy of the science of hidden defects
This is the most obscure and delicate branch of the hadith sciences. No one can undertake it except one whom Allah has granted penetrating understanding, a vast memory, and comprehensive knowledge of the ranks of narrators and the states of the chains and the texts.