It wasn't really a secret that Pinkie Pie had hidden various objects all over Ponyville "for emergency use."
Emergency party supplies. Innocent little objects. Fun stuff.
But Pinkie got more serious about it, after a few months of waking up from nightmares of Twilight Sparkle destroying ponies left and right just because they looked like Pinkie Pie.
One of those nights, Pinkie Pie yet again lay awake and imagined what might happen if a changeling or a long-lost twin showed up. Which pony who looked like Pinkie Pie would die this time? But Pinkie started thinking, maybe there's another way. What if one of the two ponies...didn't look like Pinkie Pie anymore?
That was when Pinkie Pie started hiding more supplies everywhere. Emergency eyepatches. Emergency fake mustaches. Emergency fake-mustache-and-beard sets. Emergency instant hair-and-mane dye. Emergency fake wings.
If a pony suddenly showed up who looked like Pinkie Pie, then Pinkie Pie would instantly reach for a disguise so good, Pinkie wouldn't even look like herself anymore.
That should at least slow down any reflexive lethal response by Twilight Sparkle. Would it slow Twilight enough to give Pinkie Pie time to explain? Or maybe just to run away? Running away might be safer.
For a little while, Pinkie Pie took up jogging, "to burn off some sugar and get into shape." Jogging was a perfectly normal hobby that lots of ponies did. Even Twilight Sparkle herself had taken up jogging for a few weeks to get ready for her first Running of the Leaves. Nothing suspicious about jogging at all.
But soon Pinkie noticed that Twilight Sparkle was getting better and better at teleportation. Pinkie wasn't sure if she could outrun a pony who teleported so easily.
And then Twilight had gotten her wings, becoming an alicorn. It's a lot harder to give the slip to somepony who can take to the air and look for you from above.
That was when Pinkie Pie became more serious about digging hidden underground lairs and tunnels connecting to every part of Ponyville. Airborne eyes wouldn't be much use, if you were looking for somepony who was dug in deeper than a diamond dog.
But even as she developed her secret underground tunnel network into a nearly incomprehensible maze, Pinkie Pie still sometimes lay awake at night and worried.
Reluctantly, Pinkie Pie started working on improved versions of her party cannons. More powerful propellants. More stunning payloads. Flash-bangs that would blind a pony for minutes. Precious minutes in which Pinkie Pie could beg for her own life, or try to save the life of another pony who had the misfortune to look too much like her.
Pinkie Pie started to think of new ideas for cannons that would produce more powerful, longer-lasting effects. Some of them would be dangerous weapons. Even a poison joke cannon could give Twilight Sparkle horn droop for days, knocking the alicorn out of any fight. But what kind of pony would Pinkie Pie be if she built something like that?
Pinkie Pie started to fear that she was losing her connection with something important. Was she losing the spirit of true friendship? Was she starting to get out of tune with Harmony Herself? What about just being a good pony?
Pinkie Pie remembered that Princess Celestia had invited anypony of the Mane Six to write Celestia letters about friendship lessons. Pinkie Pie hadn't worked out what the right solution to this friendship problem was, but even recognizing that she was having a serious friendship problem must be the start of some kind of lesson. At least Pinkie Pie hoped so. So late one night, Pinkie Pie wrote a letter explaining her problem. Pinkie mailed the letter to Celestia right away, before she could maybe change her mind about sending it.
The next day, Pinkie Pie received a kind reply from the Princess of the Sun herself! Could Pinkie write Celestia with more details? Pinkie Pie felt so happy, because the problem would surely soon be solved with Celestia herself taking such an interest. So Pinkie Pie answered all of Celestia's questions.
The two wrote back and forth every day for a week, and Pinkie Pie felt so privileged to bask in Celestia's love of friendship.
After the first week, though, Pinkie Pie started to wonder why Celestia seemed to be asking more questions about Pinkie's ideas for new weapons than about the rest of the friendship problem.
"Please don't make me write any more about ideas for defeating Twilight Sparkle in a fight," Pinkie Pie wrote. "I don't even want to think about that. It just makes me feel worse."
Celestia wrote back, "You've thought about your friendship and your priorities, and you refuse to hurt your friend, even if your refusal might put your own life at risk. I'm proud of you and of the friendship lesson that you've taught yourself. Also, please find enclosed a check from the Canterlot Military Works for valuable consulting services."
Pinkie Pie ripped up the check into little pieces.