Ask anyone and 'wear white' will usually be first to mind. But in this Instagram age of dramatic backdrops and OTT romance, 'propose to your partner' has had to be added to the ever-growing list of nuptial no-nos.
Of course there's basic etiquette such as not RSVPing, or turning up late to the ceremony, but with weddings now being planned with military precision, turning up with a plus one or messing with the table plan could also cause some sideways glances (at best) on the day... and we haven't even started on the hot potato that is children at weddings (them sticking their fingers in the cake, as kids are want to do, is apparently something to look out for!)
So what is the worst thing you can do at a friend or family member's wedding?
For some people, it's 'taking the attention away' from the happy couple by announcing some big news - such as a pregnancy.
Of course, since the Duke and Duchess of Sussex announced their pregnancy via Kensington Palace - three days after the wedding of Britain's Princess Eugenie to Jack Brooksbank - many royal wedding watchers are now taking a look back at the big day with an eye on Meghan Markle's interactions with guests - and resolving that the Duchess very much seemed to be discussing her happy news with family members ahead of the Palace's official announcement (the video of the conversation she has with Princess Anne being of particular interest to most).
Indeed the Daily Mail reports that the couple did in fact share the news with the family on the day of '#royalwedding2', where they congratulated by 'delighted' family members as Eugenie and Jack enjoyed their day in the spotlight.
And if it's true? For some it's a complete no-no, and seen as an attempt to 'steal the thunder' from the bride and groom.
So how long is one expected to wait to let family and friends know their happy news?
"They couldn't wait at least a week after Eugenie's wedding before announcing their news? Instead, they told the family ON the day of her wedding like wow, way to upstage the bride on her day....RUDE!" one particularly irate follower of the royal family wrote in response to the announcement on Twitter, while - it must be said a minority - agreed that the couple should have waited until after the big day to share their news with their family.
Another royal follower mused about the 'outrageous behaviour' of 'trying to outshine Eugenie on her wedding day' and said that they were 'disappointed' in Harry who "should have known better".
"Seriously... who goes to a wedding and make an announcement that their having a baby?" another fan pondered.
About 40 minutes after the tweet from Kensington Palace announcing the Duke and Duchess' happy news, mother-of-the-bride Sarah 'Fergie' Ferguson tweeted a picture of her daughter and her new husband, and posted another photo thanking her designer for her own dress in a move seen as trying to pull focus back on Eugenie. Alas, Kensington Palace, the media, and the public seemed to have already moved on - we didn't even get to see a photo of the bride in her 'day 2 outfit' (a short white dress complete with 'Mrs Brooksback' personalised pink jacket, according to the designer).
So perhaps it's more than three days after?
A week? Two?
"I’m quite surprised as Harry and Eugenie are supposed to be quite close. Perhaps she realised what was going to happen and was relaxed about it on the day but many will think that the timing was off," British Etiquette expert William Hanson told The Telegraph.
“From a strict etiquette point of view, they should not have done it, regardless of the convenience of having all the family under one roof.”
So where's the etiquette? And what about wearing white to the big day as a guest? Ayda Field (and a few others, it must be said) raised some eyebrows with her choice of suit - an off-white skirt and jacket that some found a bit too 'bridal'. Others say it's okay - she was in grey...
But how 'off-white' do you have to be? Is a pale pink okay? How about silver or cream? Surely it's okay to go powder blue - but what if the bride is wearing powder blue too?
As with everything in weddings, there will always be one person with their eyebrow raised, and someone else who didn't even know there was so-called 'etiquette' to follow in the first place.
What's the worst thing you can do at someone's wedding?
Reddit users share their 'nuptial no-nos':
- "Steal their thunder by proposing to your partner... Even when they have permission, it still seems rude as hell."
- "Show up with your kid despite being told it’s a kid-free wedding."
- "Show up 15 minutes late to the ceremony..."
- "Announce your pregnancy."
- "Make a toast if you were not invited to do so."
- "Show up with a plus one (or plus three) despite not having a plus one."
- "Let your children run around at the reception and stick their fingers in the wedding cake."
- "Call attention to yourself in any way shape or form. If you go to a wedding and no one remembers you mission accomplished."
- "RSVP that you will be attending and then don't show up - The couple will still have to pay for you as if you did come, and they probably would have invited someone else if they knew that you were unable to attend."
- "Get seconds at the buffet if not invited to do so."
- "Show up without RSVPing."
- "Show up in jeans when the invite says formal."
- "Recently attended a wedding that had three very talented photographers but everybody still had their phones up and out taking videos and pictures. Not the worst thing, but just annoying and inconsiderate."
- "Rearrange the seating chart that was worked on for a long time because you weren't happy with it."
- "Find and light the sparklers that are meant for the sparkler exit."
- "Make everything about yourself and bother the bride and groom about it."
- "Drink too much and throw up - especially if it is an open bar."
- "Compare your wedding to theirs - and tell them how it should have been done."
- "Leave straight after the dinner."
- "Wear white."