You always keep a Pinkie Promise. Aside from the principle that one should always keep their promises, the Pinkie Promise had grown into a sacred covenant amongst the ponies of Ponyville.
Applejack once came close to breaking a Pinkie Promise, but managed to explain her way out by highlighting the fact that her promise was conditioned on breakfast being eaten—since she didn’t eat breakfast, no promise was broken.
As with most superstitions, what started out as a joke quickly grew into an urban legend. In this case that Pinkie herself would come for you if you failed to uphold your end of the bargain. Because of this, a series of informal rules had sprouted up around the Pinkie Promise. Pinkie Promises were never vague—Pinkie Promise to “do anything”, and you could very well find yourself serving as a lookout in a bank robbery or running through the streets of Ponyville covered in honey. Pinkie Promises were never made more than a few days in advance. Pinkie Promise to do something months from now, and you might find yourself stuck between the proverbial rock and a hard place.
Once a pony named Blueberry had tried to circumvent the Pinkie Promise by making another conflicting Pinkie Promise; he was never heard from again.
All of this however was the farthest thing from Twilight’s mind as she happily cantered through the town square.
“Hey, Twi, got a sec?” a familiar voice called out, originating from somewhere above her.
Twilight looked up to see a familiar pegasus hovering above her. “Sure, Rainbow. What can I do for you?”
“There’s a race coming up in a few weeks, and I want to enter.”
“That’s great, Rainbow, I’m sure you’ll do really well.” Twilight paused and frowned. “What do you need me for?”
“Part of the competition is an obstacle course. I was hoping you could use your fancy magic to whip me up some gnarly obstacles to practice with,” Rainbow said hopefully.
Hmm… Some force fields, a little weather manipulation, maybe some fireworks. This could be a great chance to practice magic under rapidly changing circumstances. “Sure, Rainbow I’d love to help,” Twilight said with a smile.
“Great, tomorrow afternoon, four o’ clock in the field where we have our pet playdates. Pinkie Promise you’ll be there.”
Twilight balked at the mention of a Pinkie Promise, one of the things she had learned during her time in Ponyville was that one didn’t make a Pinkie Promise lightly. “I don’t really think a Pinkie Promise is necessary—I’ll be there,” she said reassuringly.
But Rainbow shook her head. “This is important to me, Twi. Pinkie Promise!”
Twilight sighed and rolled her eyes. Rainbow Dash could be stubborn, especially when it came to competitions, and this seemed like one of those occasions where it was easier to just give in. Besides she had nothing to do tomorrow , so what harm could come from promising Rainbow to help?
“Fine. Cross my heart and hope to fly, stick a cupcake in my eye,” Twilight said, performing the familiar gestures and finishing with a hoof covering her right eye.
“Awesome! Thanks, Twi, see ya there,” Rainbow said as she sped away, leaving a rainbow trail in her wake.
A few hours later, as Twilight found herself happily reviewing some of the finer points of force fields back at the library, she heard a knock on the door.
Applejack stood just outside the door, her stetson in her hooves as she waited for Twilight. “Howdy, Twilight. Is it alright if I bother ya for a sec?”
Twilight smiled playfully. “You never bother me, Applejack. What’s up?”
“I’m putting in a new irrigation system in the south orchard, and I was kinda hoping you might see your way to comin’ by the farm to help out.”
For the second time Twilight found herself wondering why one of her friends was asking for her help with something they were clearly more knowledgeable about than she was. “Well, sure I’d be happy to help, but what do you need my help installing an irrigation system for?”
“I thought’d I’d try one of the fancy new ones. Mind ya, I ain’t switchin’ the whole farm over, but they’ve got these new self adjusting thingamajigs now; magic crystals that regulate how much water each tree receives. And while I know everything there is to know about apple trees, I don’t know much about magic, but I sorta figured you do.”
“Sure Applejack I’d be happy to help. When are you installing it?”
“Tomorrow afternoon,” Applejack said proudly.
Twilight winced. “Does it have to be tomorrow? I already promised Rainbow Dash I’d help her with a flying competition.”
“Holes are already dug and the crystals are being delivered tomorrow. It ain’t good to leave holes open too long, roots wither and critters burrow into them and such. Look, it aint nothing— Me and Mac can figure it out ourselves.”
As she watched her friend trot away Twilight did a quick mental calculation. Applejack needed her help more than Rainbow did, and Sweet Apple Acres was more important than a race. “Applejack, wait,” Twilight called out before her friend had gotten too far. “I’d be glad to help. I’ll see you tomorrow afternoon.”
The rest of the day passed without incident, and the next day Twilight found herself pacing around the library. It wasn’t that Twilight regretted prioritizing helping Applejack over Rainbow Dash; but she did find herself nervous about the possible consequences Pinkie might dole out. It's ludicrous to think that Pinkie should be the sole arbiter of which promises are worth keeping. Sometimes you had to break a promise; this seemed like one of those times. Realizing that there would be no issue if Rainbow let her out of the promise, Twilight hurried off to find her.
“It has to be this afternoon! I need all the practice I can get.”
“Yeah, but it’s just that Applejack needs my help too, and—”
“You Pinkie Promised, Twi! If you don’t show up I’m going to tell Pinkie that you’re not keeping your promise. What do you think she’ll do then?”
Twilight sighed. Celestia had always taught her to stand by her convictions and she wasn't about to back down just because Rainbow threatened to tell on her. “I don’t know, Rainbow. But I’m going to help Applejack tomorrow. If you’d like me to help you with your race you’re just going to have to wait.”
Rainbow flew off with a “hmph”, and Twilight could only hope that Pinkie would be in a more forgiving mood.
Twilight did what she said she was going to do and helped Applejack install and calibrate her new magic regulated irrigation system. And she felt good about it. She was trotting back to the library when a shrill shriek, interrupted her journey.
“TWILIGHT SPARKLE!”
That voice unmistakably belonged to Pinkie Pie, and it would be foolish to think that Pinkie was here to talk about something else. Rather than drag out the confrontation, Twilight decided to just cut to the chase.
“I know that I broke a Pinkie Promise, but Applejack needed my help more than Rainbow did. So if you’re going to shoot me with your party cannon or drag me to some horrible dimension where birds peck out my liver, go ahead and do it already,” Twilight said and braced herself for whatever horrors Pinkie had undoubtedly prepared for her.
Pinkie giggled. “What are you talking about, Twilight? I’d never do either of those things.”
Twilight balked at her apparent pardon. “But I broke a Pinkie Promise.”
Pinkie nodded her head vigorously. “Yep, and you should say you’re sorry to Rainbow Dash, and be more careful in the future.”
“That’s it? That’s all you have to say?”
“Uh huh, you had a good reason for breaking your promise, and I know you’d never intentionally hurt Dashie.”
“What about Blueberry? He made a promise he knew he couldn’t keep and nopony’s seen him in Ponyville since!”
Pinkie giggled again. “He moved to Trottingham. You didn’t really think I’d hurt anypony over a Pinkie Promise, did you?”
“What about when you thought Applejack broke a Pinkie Promise? Your eyes were all fiery and your voice got weird.”
“Well sure, I was upset because one of my best friends lied to me. But I was only going to yell at her.”
“So all this stuff about ‘Pinkie Pie will do unspeakably horrible things to you if you break a Pinkie Promise’ isn’t true?”
Pinkie smiled one of her impossibly wide smiles. “Yeppers! I don’t know where all that stuff came from. I just think ponies should keep their promises.”
And with that, Pinkie Pie happily bounced away.