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You sit in the front row.

No ifs, ands or buts here. "Never sit in the front row unless you’ve been invited to," says Jennifer Porter, party planner and owner of . "Even if it’s a casual gathering, and save the front row for family or the wedding party." But of course, there's always an exception to the rule: "If an usher places you in the front, relish your seat and enjoy!"

You bring an uninvited guest.

Surprises during a high stress situation (say a wedding) are a no-no. If you said you'd be flying solo, . Or if the couple didn't mention that you could bring a plus one, don't assume that you can. "Every person who attends cost money for the bride and groom or their family," says Brian Worley, director of . "It is also totally awkward when your uninvited guest has no place to sit at the reception."

You're twinning with the bride.

You should never outshine the bride. Unless she gives you her stamp of approval, don't wear white (the cardinal sin of weddings). " and for the bride only," says Worley. "This is not old-fashioned."

You forget to RSVP.

How would you feel if someone just waltzed into your house for without notice? You'd probably be shocked, no? Same goes for weddings. "Most couples give guests plenty of time to respond and even provide a stamped envelope," says Lizzie Lumley, coordinator at . "Brides can't make the final arrangements until the RSVPs are in — and it's rude to keep them waiting." It's even worse to show up unannounced.

You bring a big present to the ceremony.

Whatever you do, don't make the newlyweds schlep a heavy box of kitchenware back home. "Bringing a large gift is a big annoyance for couples," says Alice Fay, Senior Catering Manager & Wedding Expert at . "They likely have a packed car to drive home regardless of any gifts they received at the wedding." That's not to say that you can't buy the couple large presents. "It’s more respectful to send to their home." Regardless, don't show up empty-handed to the reception — bring a handwritten card and hint that the actual present is waiting on their doorstep.

You walk in late.

No matter how casual the ceremony, you should still show the bride and groom that their big day is a priority to you. "Showing up late is very rude considering the amount of effort the couple went through to plan their special day," says Fay. "Guests should always account for traffic and potential public transportation delays to ensure they are on time." Because you'd feel awful for missing the bride's big entrance, wouldn't you?

You buy a gift that's not on their registry.

Before you get creative, take a peek at . "The greatest gift in the world is something the couple has already indicated they want," says Lea Berman and Jeremy Bernard, authors of "." But if the gift price points are out reach, pitch in on a larger gift with other guests. There's always a way to meet your needs and the wishes of the couple.

You take your own pictures and post them on social media.

We get it: You really want to show off how much you love the newlyweds with their adorable Instagram hashtag but experts say it's best to wait until after they say "I do." "Guests should refrain from taking photos or videos during the ceremony, if the couple asked them to," says Shawna Orwoll from "Religious or not, a wedding ceremony is a sacred moment for the couple and should be treated accordingly."

You interrupt the newlyweds.

Every couple deserves one minute to enjoy the meal they perfectly selected (and paid for) on their wedding day. "It always amazes me how some guests will go up to the newlyweds and interrupt them for a selfie or to ask them to come visit their table," says Hovik Harutyunyan, owner of . "The couple is there the whole night. They aren't going anywhere so why bother them during their meal?" Exactly.

You play "The Price Is Right" all night.

It's a fact that weddings are insanely expensive but it's not your job to play detective all night and figure out how much it costs. "The venue is spectacular, the and the bar top of the line, but to openly speculate on the costs is trés gauche," says Jodi Smith, founder of . "Instead enjoy the fact you have been invited to such a fabulous affair and leave it at that."

You question the couple's no-kids policy.

If the invitation says "adults only," then follow the rules. It's as simple as that. "In some cases, children can ruin the ambiance of an elegant, mature event," says Harutyunyan. Plus, you wouldn't want to be the one responsible for putting the happy couple in an uncomfortable situation on their special day. "Other guests who followed the rules may wonder why your child was given preferential treatment — even though that wasn't the case at all."

You give an inappropriate toast.

Save the college horror stories and funny dating escapades for your bachelorette party. "Toasts should be short and sweet," says Harutyunyan. "Don't talk about exes, dysfunctional family dynamics or vent about unrelated drama." Remember: This isn't your wedding day so make sure the toast is directed at the couple.

You make a beeline to the dessert table.

As tempting as it may be, don't dive into to until the couple makes the first move. "Even if there are desserts available, pay attention to the wedding staff and couple," says Tiffany Hayden, owner of . "The couple will still cut the cake and it looks bad in photos if half the table is picked over and empty." Good point.

You don't sit at your assigned table.

The saga of choosing who sits where is already difficult enough so don't add more fuel to the fire on the wedding day. "I had an issue at a wedding where the bride's uncle didn't like that his cousins were at another table," says Lumley. "He was putting on such a show that the parents of the bride were willing to move to please him. Eventually, the bride and groom got involved, which ultimately put a damper on the whole day." Just like grade school, sometimes you have to suck it up and sit in assigned seat. You'll be okay.

You drink one too many glasses of bubbly.

This goes without saying: Nobody has time for a drunk party guest — and the open bar isn't to blame. "We all love an open bar, but as a guest it's important to understand your limits so you don't embarrass yourself or the couple," says Hayden. So, drink responsibly and cheers to the happy couple!