Many agree that these slights usually aren’t deliberate. “Most of the time not bringing a gift is not out of malice,” said Anja Winikka, the site director for the Knot. “People forget. They go to a wedding over the weekend and they move on. Your wedding is not their wedding.”

Still, when it happens, it can sting.

Kate Sawyer, 28, a project manager in Medford, Mass., is lacking three gifts from her October 2012 wedding. While the errant items technically fall within the oft-cited one-year window (more on that later), she is insulted. “Before your wedding you look at these etiquette books and these old traditions, and it’s kind of fun,” she said. “And you’re so careful and spend so much time opening gifts and writing down who gave what to you so you can send them a proper thank-you note. And then you don’t get a gift and it’s like: ‘Really? How do you do that?’ ”

There are levels of annoyance, depending on who the guest is. Less affluent guests, for example, may be given more leeway than wealthier ones. As may be the case for men, both single and married. For example, Wendy Kaufman, 54, who gained fame as the Snapple Lady, has never forgotten the three people who did not give a present at her lavish, 300-guest wedding in May 2004.

She gives a pass to one of the offenders: the 22-year-old son of Ms. Kaufman’s college sweetheart, who was barely able to support himself and brought no gift. On the day of her wedding, in fact, Ms. Kaufman and her groom ended up peeling off five $100 bills for him. “We’ve been kind of surrogate parents to him, and he needed the cash,” she said, chuckling. While she didn’t mind the lack of a gift, she was a little miffed that he never wrote a card. “That would have meant more to me,” she said.

Ms. Kaufman is less forgiving with her parents’ millionaire friend who “drank her face off at the wedding” and never gave a gift. And she can hardly be in the same room with another couple, business colleagues, who didn’t attend the wedding but did not send a present, either, despite the generous gifts Ms. Kaufman and her husband had given them for their engagement, wedding and first child (all three gifts were from Bergdorf Goodman).

But it’s not the husband who angers Ms. Kaufman; it’s the wife. “I don’t hold the man responsible,” she said. “He was out working, and she was a stay-at-home mom. I know it’s sexist, but she should know better. When I see her at events I want to blurt out: ‘You cheap jerk. How can you sit here and have a conversation with me?’ ”