More than likely, you had an error or a warning returned by eBay during a listing session. And eBay's warning told aW2000 the auction didn't list when in fact it did.
Have you had an listing errors or warnings lately?
And yes I have experienced the same thing before, and it was just a week ago, I think. eBay was throwing server errors. I'll have to go back and check my listinglog files on the server to see which error it was. But eBay API was returning the Auction didn't list when it really did. I caught it because I knew how many auction lots I listed and I could see the Listing confirmation email notification count was the same number. So I got suspicious and checked. Sure enough the Auction lot listed.
The only thing you can do with your buyer is send an apology. Offer a replacement item or send them a small gift. Buyers usually respond positively to an honest explanation. They are people just like us that have stuff happen to them.
Let me know about the errors or warnings when listing.