But the expert is the one who can know if the requirements are sensible.

In requirements gathering, the whole job is to hear people's attempts to describe their problem and figure out what problem they actually have.

By definition they don't have your expertise, or they wouldn't need to talk to you.

So, of course they will say contradictory things and use terms completely incorrectly - they cannot do anything else. That's why they hired you.

The expert in this scenario gets hung up on their incorrect language and gets flustered and stymied, telling them "What you want is impossible!"

What they _said_ is impossible. It's our job to persistently, patiently, calmly help them understand their needs, without judging them for needing our help.

I'm not particularly good at it, but I understand the mission.