Of Tiffany Schutt’s 250 wedding guests, one surely stood out.

Not only was she not invited, but the young guest, a relative with whom the couple wasn’t particularly close, turned up in a white dress — and a short and sexy one at that.

In fact, she was one of five uninvited relatives whose names were added to invited guests’ response cards. Schutt, who married in Indianapolis, was flattered but also in disbelief that they so badly wanted to attend.

"We are very laid-back, thankfully, so that day I took it in stride," she said. "It just seemed not to be the best etiquette."

When it comes to manners, experts say wedding guests do well overall but are still causing headaches on a few fronts.

"The No. 1 thing that I hear about from frustrated brides is guests not RSVPing, not RSVPing on time or RSVPing for more than one person," said Anna Post, great-great-granddaughter of Emily Post and co-author of the upcoming new edition of "Emily Post’s Wedding Etiquette" (William Morrow). "It’s all about the RSVP."

Blame it on the relaxed culture, busy lives or the hope of a better Saturday night offer, but some people just can’t get it together to mail the response card back. And don’t get brides started on the guests who say they will attend, only to end up as no-shows.

"People have gotten cas­ual about this," Post says. "When it comes to the wedding, they think it’s not a big deal, I’m just one person, it’ll be fine. And it’s really not. It’s a lot of money and a lot of stress for the couple."

Brides and grooms might want to add a cushion of a week or so before they have to give a head count to the caterer or venue so they can chase people down.

"The key is to keep the frustration out of your voice," Post says. "It can’t be those sweet words with a nasty tone because that defeats the purpose of wanting to invite them in the first place. You can’t say, ‘I’m so annoyed with you, you’re uninvited.’"

New York wedding planner Marcy Blum suggests adding an enclosure with the invitation listing an email address people can use to RSVP or ask questions.

Brides put thought into addressing their invitations, which spell out exactly who is invited. When guests treat the RSVP card like a write-in ballot, a phone call is in order, Post said, so they understand that their date or child was not invited.

Some parents don’t like leaving their children home, but experts urge couples to resist such pressure.

"Do not cave to this, because it’s completely unfair to all the guests who do find child care and respect your wishes," Post says.

Besides RSVPs, another etiquette trouble spot is smartphones — in particular, those guests who crowd the aisle to take pictures and then post the images online before the ceremony is even over, scooping the couple’s wedding photographer.

"A bride is very particular about how she looks at her wedding, and she does not want the photo that isn’t the most possible flattering photo to be all over Facebook before she gets to look at her wedding shots," Blum said.

Photos aside, phones detract from the solemnity of the ceremony.

"If you’re so busy tweeting and Instagraming, you’re not paying attention," Blum said. "It’s almost a cosmic faux pas. People want your energy with them and focused on what they’re doing."

Blum, who has planned the nuptials for notables like LeBron James and George Soros, says couples can try to head this off by including a note in the welcome gift or on the program. The officiant or best man can make an announcement, or you can place a sign at the entryway.

Keeping guests from snapping away at the reception, though, may be a losing fight, Post says, though couples can ask guests to refrain from posting the photos online.

"I would pick and choose your battles," she said. "The ceremony is the right place to focus."

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Lisa A. Flam, Associated Press