I usually go with, “Your mama know what you do for a living? Is she proud of her son?”
As the website says, the first 6 digits represent the issuing entity. Those are assigned by VISA or MasterCard central. All they’d need to do is publicize to the relevant standards body that issuing bank # e.g. 412345 will never be given out by VISA and then folks like IT testers can be assured that any card starting with those 6 digits cannot possibly duplicate a real card.
Just as the US phone industry will never actually issue a 555-xxxx phone number, so those may be safely used for examples, in media, etc., without inviting spamming some real person.
It’s unclear to me whether @running_coach’s cited website does any such thing or just generates something that starts with the correct card type designator, has the correct length, and passes the check digit test. If the latter, then they very probably can and have generated numbers that duplicate currently live or previously-live numbers.
The problem would only arise when somebody tried to process a live payment transaction with such a number. If someone is just running such numbers around in the point-of-sale computer system they’re developing it won’t matter that some of them may real live numbers.
Yeah, I tried that once too. The guy actually called me back, cussed me out, and said his family was proud of him.
The response here would be “Do you think your mother would be proud of you if she knew you just called me on the phone and told me to fuck my mother and sister? You know she wouldn’t. She would tell you she didn’t raise you like that. And she would tell you not to work a job that made you say things like that.”