I once had a QA engineer file a bug saying they couldn’t do negative testing since negative numbers were converted to positive.

The function took an unsigned integer. Took a lot of explaining to get them to understand that negative testing isn’t necessarily negative numbers.

Even more real scenario: The first real visitor isn’t even a customer but a bored teenager who says nothing at all and instead takes a piss on the floor. (Anyone who ever published anything on the internet knows this scenario.)