eBay has been rolling this out to more sellers. When you successfully accept a buyer's offer it sometimes goes to Pending, waiting for the buyer to pay. Go to your active listings page in seller hub and view all offers for that item - see if it's in Pending status. If you see it there, it's not an error, it's something eBay has been testing with accepted offers. They remain in pending status while the item is available for anybody else to purchase until the buyer pays. It's one of several things eBay is testing to cut down on non-payers and the inconvenience to sellers when buyers lose interest after an item ends, and seller has to start from scratch when they relist. If the buyer does not pay within a certain amount of time it will time out and be removed from the pending section. If the buyer pays then it will become an official transaction on your orders page @samiam60 .
