Yes, even before you arrive at the wedding ceremony, you can make mistakes that can drive an engaged couple crazy. Luckily, it's pretty easy to avoid committing these offenses.
Send in your response card late -- or not at all.
If you receive a wedding invitation with a response card, make note of the reply-by date. It's usually a few days to a few weeks before couples have to give their final head counts to venues and caterers. If you don't return the card, expect a call (or a text or an email) from the couple or a member of the wedding party. It may not seem like a huge deal for them to get in touch with you, but it's a pain because you're likely not the only guest who couldn't bear to write your name, check a box, and drop an already stamped envelope in a mailbox with four to six weeks' notice.
Change your response after the reply-by date.
Brides get it -- things come up. But you should understand that your plate may already be paid for. It's not so egregious to alter your response before the RSVP date (just don't change again!).
Forget to fill out your name on the response card.
Most couples know to number the backs of their response cards and have each number correspond with a particular guest they've invited (on the off-chance that person neglects to write his or her name on the card). But not all couples do the numbering trick, and the process of elimination can't help if multiple guests return blank cards.
Send a wedding gift without a card, or without signing the card.
For the same reason, this is confusing. Registry items purchased online don't tend to list the gift-giver's return address. The couple probably would love to thank you for the thoughtful gift, but how can they if they don't know it was you who sent it?
Send a wedding gift to an address the couple didn't select.
My friend specified that wedding gifts should be sent to her and her fiance's shared home address. One guest thought she had a better idea: Send it to the bride's parents' home as was the norm in the past -- except the bride's parents moved. The gift almost didn't ever make it to the couple because of issues with setting up the forwarding address (it arrived eventually, worse for wear).
Ask to bring a guest.
If you're friendly with a soul other than the bride or groom at an upcoming wedding, don't ask for a plus-one. To stay within budget, the couple may have decided that unattached folks with other friends at the wedding don't get dates. If you won't be happy going solo, don't go to the wedding. The same rule applies to asking to bring children. If you can't or don't want to get a babysitter, decline the wedding invitation. If the couple absolutely wants you there, they may ask why you can't make it and offer to allow you to bring a date or your kids -- feel free to take them up on it if they do. But in most cases, it's better not to bring this up if they don't.
Ask a million questions of the bride.
If you have dietary restrictions or allergies, let the couple know so you can eat at their wedding. Other than that, there aren't too many subjects that warrant mentioning. There's likely info on how to get to the venue in the invitation (or, you know, on these here Interwebs); if the dress code isn't specified on the invite, women are probably safe to wear cocktail dresses and men are likely good to go in dark suits if it's a Saturday evening event; and you don't need to know the exact end time of the reception because you can leave (almost) whenever you'd like.
Complain about the wedding to the couple.
Someone groused about my "Memorial Day Weekend" wedding to me -- but we were getting married the Saturday after the holiday. Even though that complaint was unfounded, brides and grooms choose the wedding date and location that work best for them -- end of story. Again, simply decline if you can't make it.
Anything you'd add to this list?