Please, unless the bride specifically requests it, do not wear white to a wedding as a MOTB, wedding guest, or member of the bride's crew.
If a wedding invitation specifies a certain level of formality, you are expected to adhere to the dress code, even if you do not wish to wear black tie.
There is nothing more humiliating than your friends asking, "Who is that person?" for years after viewing their wedding images.
As much as possible, don't go against the wishes of the newlyweds by getting a "unique" wedding gift.
It's best to keep those humiliating anecdotes PG-rated when it's a family affair, since otherwise things can turn awkward very quickly!
Keep your phone away from the altar, especially during the ceremony. To be a guest, not a photographer or videographer, is what you were asked to do.
They should be congratulated and greeted, but you shouldn't take up too much of their time when they're attempting to work the room.
This guideline should go without saying, but if you were not invited, you should not go. And if you get an invitation without a plus-one, do not bring an uninvited guest!
Flirting at the singles table is perfectly appropriate if you're single and looking for love.
If you show up late and follow the bride into the reception, you'll seem sloppy. Don't be late for the ceremony or reception, and don't be late for the reception either.