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So you're telling me that I've been using a test card for the past 5 years? I hope not.

The two numbers listed on that site are test numbers. 5424 is a very common MasterCard prefix.

I'm reasonably sure that's not true. Perhaps the whole 16 digit sequences listed there is a test number.

You are right, I was wrong. It's the full number that's a test number. 5424 0000 0000 0015

There are the only 4 numbers in the results. Although I do see a few in this format XXXXXXXXXXXX2376.

(Edited to remove actual numbers in case they're not test numbers)

My actual MasterCard # starts with 5424.

According to the link that you posted, 5424 0000 0000 0015 is a test number. That's not the same number that I'm seeing in the Google results.