Years ago when I worked at Petsmart (obviously only sale freshwater tropical fish)

A couple comes in " Are all these fish tropical fish?" they asked. I answered "Yes" They proceed to purchase a Bala Shark and a few other cheap fish and leave. 2 hours or so later they return with dead fish. " You told me these were tropical fish and when I put them into my saltwater they died instantly" then I replied "Tropical- does not meen Saltwater, simply that they are from tropical regions of the planet or closer to the equator." They understood after that and the woman said to the husband " I told you they didnt look like saltwater fish"

I will never forget it, LOL.