A muʿallal report has an outwardly plausible appearance of soundness but contains a subtle and consequential defect exposed by critical investigation. It is connected to wahm: a transmitter has made an error in a chain or wording which only comparative study may uncover.
«وَتَحْصُلُ مَعْرِفَةُ ذَلِكَ بِكَثْرَةِ التَّتَبُّعِ وَجَمْعِ الطُّرُقِ»
The defect is recognized through extensive investigation and gathering the different routes.
Example: one transmitter supplies a connection to a report that stronger routes preserve as mursal. Counting chains alone is insufficient: narrator precision, specialization with a teacher, and other evidence matter. An openly visible disconnection is not the same as a hidden ʿillah.