Including registry information with the wedding invitation.
It's like engraving, "I expect a gift!" on the front of the invite in bold type. Of course you expect a gift (even though it's not required) but try not to make it look so obvious. The only places where it's appropriate to list your registry details are on your wedding website, wedding app, and bridal shower invitation.
Not sending thank-you notes.
It would be an epic fail not to acknowledge your wedding gifts with a thank-you note. (It doesn't count if you thanked someone at the wedding.) We know It can be a tedious chore to handwrite dozens of notes when you'd rather give your full attention to Master of None, but when people are gracious and give you a gift, you need to be gracious back and send a written thank you.
Spreading the word that you only want cash, not boxed gifts.
Really? It's never good manners to put limits on the types of gifts you want, and it's especially tacky to plead your case for the cold, hard stuff. Be grateful people want to give you gifts at all.
Cutting someone from the guest list after sending them a save-the-date.
This isn't The Voice, where people getting eliminated is part of the plan. If you're not 100% sure how many guests you can ask to the wedding, hold off sending save-the-dates until you do.
Not inviting a significant other to the wedding.
We get it—you have a strict "no plus ones" rule. But if two singles have a serious relationship, both should be invited to the wedding even if they're not living together or you don't like the partner.
Texting a last-minute invitation to someone on your B list.
Knowing they didn't get a printed invite when everyone else did will be upsetting for someone who considered herself a close friend. Getting a last-minute text invite from you will totally embarrass her.
And for the guests:
Wearing a white gown when you're not the bride.
Even if the bride is wearing pink, hasn't been chaste since freshman year of high school and is known for her all-black wardrobe, she owns the color white on her wedding day, whether she wears it or not. Stay away from it!
Waiting a year to send a wedding gift.
You're too busy, financially challenged or both, and you've been putting off putting out for a gift so you invoke the "one year" rule. Which doesn't really exist. Yep, that's right—it's an often-cited myth probably invented by a wedding guest who showed up for the big day empty-handed. Buy a gift within your means and do it before the organist starts playing "The Wedding March."