Zayd says, “I heard this hadith from Bakr.” Now consider two different responses.
Response A: Bakr says, “I certainly never narrated it to Zayd.” This is categorical denial and creates a contradiction concerning that act of hearing.
Response B: Bakr says, “I do not recall telling it to Zayd.” This is failed recollection; an established, reliable report from Zayd need not fall solely because Bakr forgot.
The critic also checks Zayd's reliability, evidence of actual hearing, and alternative routes. “The teacher did not recognize it” can conceal two analytically distinct scenarios; the teacher's precise words matter.