I got an order for a book on my shelf.  I accepted the order and immediately printed out the wrapper but didn't mark it mailed until the next day after I put it in the mailbox.  About 2 hours later I received two PM's together from the recipient.  The first one said "I thought you were going to cancel this order because it is a paperback not a hardcover like I wanted" and the second one said "I see now that the order I placed was for a paperback".  I never received a previous PM about cancelling an order because it was a paperback.  I wrote back and explained that to the recipient, and I know it's too bad for him since I already mailed the book.  My question is could he possibly have PM'd someone else who did cancel the order and then I was the next in FIFO line?  Would that explain why he thought he PM'd me about this book?  I know it doesn't matter since he's stuck with the paperback now anyway but I'm just curious. 

It sounds like that is exactly what happened. If the sender cancelled the transaction then it rolled to the next sender, you. The person who placed the request is the one who had to cancel so it did not roll over to the next one in FIFO. Oh well, they requested a paperback and they are receiving a paperback...you did nothing wrong and have nothing to feel bad about.