This is perhaps a borderline example. I heard this story from BCnrd a few years ago, so I hope it hasn't been too mangled in my head since then.
One day, Ken Ribet got a phone call in his office:
- "Is this Professor Ribet?"
- "Could you tell me what one tenth of one percent means?"
- "One part in a thousand."
- "Are you willing to give that answer under oath?"
It turns out that someone around Berkeley had rented some property, and the rental contract specified that the landlord could not increase the rent by more than one tenth of one percent each year. When the landlord tried to raise it by more, the tenant sued.
At the trial, the tenant's lawyer called in the expert witness. They swore him in, asked him about his job and his qualifications, then came the key question:
- "Professor Ribet, what, in your expert view, is one tenth of one percent?"
- "It's one part in a thousand."
Before he could give a rigorous proof, the landlord's lawyer said, "Objection! Anyone reading the contract can tell that it obviously meant one part in 10." The judge agreed, and threw out the case.