Wahm is an erroneous assumption or attribution in a specific report. It may not be obvious on first reading but emerge through comparison of routes and subtle supporting evidence. A narrator may be personally honest and generally accurate while making such a mistake on one occasion.
Schematic example: A preserved route is mursal, but another narrator mistakenly reports it as connected; or one transmitter is wrongly substituted for another. Judging which route is preserved requires comparative evidence.
This cause leads to the subject of the muʿallal (defective) report. Every variant is not an ʿillah: the critic must identify the mistake and the reliable evidence revealing it.