The Love Car Displacement - S4-E13
Corrected entry: At the end of the show Sheldon states on the walkie-talkie, "I spy with my little eye, a ferrous metal," Later he reveals that what he spied was the aluminum rims of a police car. Aluminum is a non-ferrous metal - definitely not a mistake Sheldon would make.
Correction: Sheldon does say a non-ferrous metal.