I have occasionally heard from customers who were trying to pay by credit card on paypal but kept getting a message saying they had entered an incorrect or incomplete number, even though double-checked everything and may even have tried more than one card.
I have had this same problem myself - after initially making an error I had to do something non-intuitive to get the payment to go through, but I don't remember what. I do remember thinking, I should write this down in case I have to walk someone through the process!
Most of the time (from experience with other processors), it's been user error. They SWEAR they typed in the correct information, when in fact they forgot a digit or transposed a number or mixed up the expiration date. And yes, I'll even admit to doing the same thing myself. :)
I spent a few minutes trying to reproduce the problem and entering the card # with spaces or dashes is probably what did it. The need-to-do-something-odd to re-enter an invalid # no longer appears to be a problem.
I can't believe Paypal can't be bothered to prompt customers that you need to enter the card # without spaces! I've sent an email suggesting that they provide a prompt. I can put a note on my own site, but that will be 2 screens away from where they need to enter the card info, and it will look clunky at best.