> When the client tells you what the cause is, he's wrong.

using a software analogy, I've seen plenty of bug reports where the reporter did some research on his own that turned out to be determinant in the issue resolution. You have training, inside knowledge of the system that's failing, but you see plenty of issues everyday, cannot dedicate to each one the time they would deserve. The reporter instead is motivated to reach a solution and has plenty of time to research it properly. Discarding a potentially fundamental input just because it comes from someone supposedly untrained would be a shame.