Straight: Amber is supposed to marry Sapphire, but on their wedding day, she ditches him in favor of her True Love Jade.
Exaggerated: Princess Amber is about to marry Prince Sapphire, but has fallen in love with errant-knight Jade. Torn between her duty and vows and the man she truly loves, she flits back and forth, only deciding while standing at the altar to choose love over all other considerations. When the priest intones "Speak Now or Forever Hold Your Peace", she turns to Sapphire, apologizes, and runs off into Jade's waiting arms.
Justified: Amber is being forced to marry Sapphire.
Amber ditches Jade in order to marry Sapphire, arriving at the church at the last possible minute.
Sapphire ditches Amber on their wedding day.
Subverted: Amber has been second-guessing her commitment to Sapphire... but when the opportunity to "Speak now" comes up, she remains silent.
Double Subverted: ...But then turns and runs out right as the priest is saying "Then, by the power vested in me..."
Parodied: Amber and Sapphire have secret lovers, and when the big day comes...
Zig Zagged: Amber ditches Jade on or hopefully beforetheir wedding day and goes back to Sapphire. Whether or not he actually takes her back is a different story.
Averted: Amber either sticks by Sapphire's side or calls off the wedding before the big day.
Enforced: Amber and Jade are the Official Couple; Sapphire is just a Romantic False Lead, and the writers want to drag things out before the inevitable confession as long as possible.
Lampshaded: As soon as Amber starts going through the motions, the officiant sighs and mutters, "Not this again," having seen his fair share of Runaway Brides over the course of his career.
Invoked: Jade tries to goad Amber into running away from Sapphire.
When Amber declares her love for Jade, he rejects her for any number of reasons, leaving her quite SOL.
Or, when Jade interrupts the wedding at the SNOFHYP part, Amber tells him she chose Sapphire and that he must leave if he can't respect their love (or has him thrown out for interrupting the ceremony in the first place).
Discussed: When Amber tells Sapphire she cannot marry him, he angrily asks why she couldn't have brought this up before the wedding.
Conversed: Amber and her beau (be it Jade or Sapphire) are watching a movie that features a Runaway Bride/Groom and discuss their opinions on the matter. If they're not already married, they might come to an agreement not to be so crass at their own wedding.
Amber and Jade are forced to leave town because nobody in their social circle will let them live down what they did to Sapphire. Sapphire is furious and turns public opinion against them, painting himself (rather justifiably) as the victim of Amber's thoughtless flight of fancy.
There's also the matter of who pays for the lavish-but-aborted wedding party...
Jade comes to doubt Amber's commitment to him; given that she ditched a guy on their wedding day, can he really trust her to stick with him through thick and thin?
Reconstructed: Amber and Jade are driven out of town... in the pilot of a Walking the Earth series.
Played For Laughs: Amber is notoriously fickle and has a habit of leaving men at the altar to chase after some new flame. She has an entire closetful of wedding gowns that she's collected from this habit. Jade is fully aware of this and expects to be ditched at their wedding; imagine his surprise if she stays...
Played For Drama:
Sapphire honestly loves Amber, but she's torn between her feelings for him and for Jade, uncertain of which one is her True Love and which is just a passing crush. When she decides to ditch him at the altar, Sapphire snaps and attacks Jade in a jealous rage.
Following the event, whether Sapphire loses his cool at the aborted wedding or not, he becomes an emotionally scarred man unable to form lasting relationships for fear of being hurt again, possibly prone to bouts of depression and/or alcoholism. This could very well be the setup for a Romance movie itself; see The Wedding Singer.