Another one was many years ago when the guy asked if the gas operated shotgun he'd just bought took regular or unleaded and asked where the "tank" was, and yes, he was serious...
Not at a gun shop, but my boss (for a very short time) years ago bragged about his .50 cal Desert Eagle. Said it was really accurate, but a hassle to shoot because it took CO2 cylinders. I asked if it was a BB gun, nope, "it shoots 50 AE".
Figured out later he'd heard they're gas operated and thought that meant it ran on CO2; obviously he'd never owned one.
