According to the leading opinion among the ḥadīth specialists, an anomalous report is one narrated by a trustworthy, generally accepted transmitter that nevertheless contradicts someone who is more qualified than he is.⦗1⦘ Ibn al-Ṣalāḥ explains this point in his Introduction, p. 86:

“If a single narrator transmits something on his own, the report is examined. If what he alone has transmitted conflicts with the narration of someone superior to him in retention or greater in precision, his solitary report is deemed anomalous and is rejected. If, on the other hand, his narration does not contradict what others have reported—rather, he alone has related it while no one else has—then one must look at that individual narrator himself. Should he be upright, a memoriser, and trusted for his accuracy and precision, his solitary report is accepted and the mere fact that he is alone in narrating it is not deemed harmful. But if he is not of those whose memory and accuracy inspire confidence with respect to the very report he has singled out, then his isolation undermines him and removes the narration from the category of sound reports. After that, the report can occupy various ranks according to the circumstances: if the person who stands alone is not far below the level of a reliable, precise memoriser whose solitary reports are accepted, we regard his ḥadīth favourably and do not relegate it to the class of weak reports; but if he is far below that standard, we reject what he alone has transmitted and list it among the anomalous and objectionable reports….”

Anomaly may appear in the chain of transmission or in the wording itself. Each type has many examples, some of which will be pointed out in their proper places, Allah willing.