Contradiction of reliable transmitters is a material conflict between one report and a better-preserved transmission from trustworthy authorities. Not every difference in wording is a harmful contradiction: variants may agree in meaning or represent compatible additional detail.
For example, one transmitter gives an identifiable element differently from several stronger, independently preserved routes. The critic then weighs the evidence and locates the mistake.
The topic connects with the earlier distinctions between maḥfūẓ/shādhdh and maʿrūf/munkar, and with later categories of isnad alterations. Number of routes, precision, specialization, and transmission relationships all matter.