With Stripe there is a test mode - to allow you to run test cards.
These cards get saved to the customer.
When you're ready you move your system to LIVE by putting in your live Stripe API keys.
If you then try to use the same customer to run a LIVE transaction you will get an error like this:
"Your card was declined. Your request was in test mode, but used a non test (live) card."
So you must create a new customer before you can run a LIVE card transaction. Or a customer that has not had a TEST card saved against it.