A Changeling relives Too Many Pinkie Pies over and over.

7804445 A mirror pool is actually a changeling hive in disguise.

When Pinkie Pie used it, changelings pretended to be Pinkie Pie so that they could infiltrate Ponywile with a firm alibi. They would have gotten away with it too if it wasn't for that pesky bookworm.

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I remember reading a story where the mirror pool/pond is a disguised Changeling hive.

Edit:If only I could remember what it was called.

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I found it. At least, it's something similar to the idea.

7804531 Damn it, who made 8 billion people on this planet? How can I possibly think of something that the rest 8 billion haven't thought of before?