A central form of tadlis al-isnad occurs when a narrator cites a teacher he did meet, but did not hear the particular report directly from that teacher, using a nonspecific formula such as “ʿan” or “qāla.”
Hypothetical case: Ahmad heard many reports from Khalid, but received one particular report from Zayd who had heard Khalid. If Ahmad says “from Khalid” without making Zayd's role clear, tadlis al-isnad becomes a concern.
The decisive question is not simply whether Ahmad and Khalid ever met. It is whether Ahmad heard this individual report from Khalid. A general meeting does not prove direct receipt of every report attributed through that relationship.