Anyone else here think that Derpy has a prosthetic eye that never seems to point in the right direction? I do. After pulling a particularly long shift at the post office, Derpy flies home, but not before crashing into a tree. She is so exhausted she doesn’t notice that her “glass eye” fell out, and just flops into bed without another word.
Early next morning, Angel finds it and takes it to Fluttershy. She recognizes the iris color, and talks to Derpy about it. Before she can explain, a ruckus in town causes them both to rush to the town center, where Sugarcube Corner has just almost burned down due to a gas-leak/explosion. The other ponies tell Fluttershy and Derpy that they saw Pinkie being taken to the hospital. They and the rest of the Mane-6 ponies visit her, relieved that she only has minor bruising.
It’s only then that they notice that Derpy is, rather conspicuously, missing an eye. She flees out of the hospital in embarrassment/shame/fear (after grabbing an eyepatch Pinkie had with her for “eyepatch emergencies”), and Fluttershy flies after her to try and comfort her. This leads to the other Mane 5 coming up with theories on how Derpy came to lose that eye. Each theory is its own little story, “Rashomon-style”.
From Rarity’s “film-noir” style story, to Applejack’s war/adventure story, to Twilight obliviously telling (and nauseating) everypony about this one book on ocular diseases and injuries she was reading the other night, especially this one called “severe eye proptosis”, where the eye itself, due to head trauma, actually pops out of the socket, and Pinkie’s story about sacrificing it to a well to gain super-knowledge, or something like that, mentioning that someone his great-great-grandmother Lokie Pie knew did that very same thing.