A few weeks ago in early October I drove 80 miles through the dark and rain and road construction and buffeting winds to sign books at a conference.  I made it to the conference.  My books didn’t. Turns out two numbers in the zip code of the address of the conference had been transposed, so UPS hadn’t managed to deliver the books in time for the morning signing. 
I’d heard this horror story from other writers, of having no books at a supposed signing, but this was the first time it had happened to me.  I have my own horror stories of signings, of getting the date wrong and missing a signing, of signings where no one shows up to buy a book, of sitting next to an author whose has long lines of folks buying multiple copies of his book while I contemplate taking up knitting. You never know what a signing will bring, but you show up and hope your books do as well.
I was assured that the books should arrive momentarily.  They didn’t make it by the time I had to leave to drive the eighty miles back home, but I spent the hour saying hey to authors I hadn’t seen for months and chatting with my table mate whose books were in the same UPS shipment.  We covered everything from stop action animation to bungee jumping, rappelling, and white water rafting, scary and not so scary movies.  I even managed to photo-bomb a picture. 
I vowed to always bring some of my own books with me from now on to signings and drove home past trees just beginning to turn, listening to weather reports of snow before morning. Minnesotans know that when it comes to weather, anything is possible.
Writers know this, too.  We dwell in possibility, both good and bad. That email you just opened or that phone you just answered could be the news that a book is out of print or an offer to turn your story into a play.
That knock on the door?
It might be opportunity.
Or it might just be the UPS driver with a box of books sent to the wrong zip code.