«وَمَتَى تُوبِعَ السَّيِّئُ الْحِفْظِ بِمُعْتَبَرٍ»
When a narrator with poor memory receives support from a credible transmission.
The supporting route must be assessed for its own reliability and type of defect. The passage particularly discusses corroboration by a comparable or stronger source; the mere addition of a still weaker path does not necessarily establish meaningful support.
Example: one truthful but imperfectly precise narrator gives a report, and an independent narrator of comparable reliability supports it. If common dependence on the same mistake can reasonably be ruled out, the overall report may gain strength. The key is the quality and independence of the evidence, not the bare number of names.