There’s a knock at the door. You answer it. Oh, look. It’s your nephew John with — surprise! — his new girlfriend, Betsy. Twelve carefully arranged place settings, 13 guests. What now? It depends.

If you prepared 12 of everything, “you may be tempted to offer Betsy your Cornish game hen, selfless hostess that you are, but resist that urge,” says Amy Nebens, author of A Gracious Welcome (www.amazon.com, $20). “You’ll make your guest feel more awkward.” Instead, split John’s hen (or chop or tart) between two plates and explain away the awkwardness with humor: “I’m afraid I made only 12, but I know John will be happy to share!”

If Betsy’s a vegetarian and your menu isn’t, “focus on what you do have rather than what you don’t,” says Nebens. “Offer her a few dishes, and explain how each was prepared—for example, without bouillon.” In a pinch, raid your cupboard or freezer for a pilaf or some other veg dish your microwave can whip up.

If you’re hosting a gift exchange, “there’s not a lot you can do about coming up with another gift,” says Nebens. “Just make sure Betsy feels like a part of the group by inviting her into the circle and encouraging everyone to tell the stories behind the gifts (‘Claire started collecting porcelain cows…’).”

A few days later, says Nebens, you might tell John, “I was thrilled to meet Betsy, but next time do give me a heads-up — you’ll get your own Cornish hen that way!”