Derpy had become a staple in the find-the-easter-egg game people played when a new episode was aired. She usually wasn't difficult to spot, and it was a joy when you did- the show staff apparently loved her as much as we did, and almost everyone enjoyed the subtle nods, but it was always exactly that: subtle. This episode completely broke that practice by including a fully animated and voiced version of Derpy, played by none other than Tabitha St. Germain, to start the plot train rolling for Applejack's trip away from Ponyville.
The reaction was instantaneous and incredible. Rooms streaming the episode were flooded with people wondering if they had indeed heard Rainbow Dash pronounce that name correctly. Whatever else happened episode-wise in the few minutes after that was lost in the excitement.
What happened in the days after is still disputed. As far as we are aware, an undetermined source contacted The Hub with the complaint that Derpy's portrayal in her scene was offensive to those with mental disabilities. This claim was made in spite of the fact that the episode's writer, Amy Keating Rogers, has a son who is similarly disabled, and is an advocate for the disabled. Regardless, the episode was promptly pulled from the iTunes repository and only reappeared some time later with an edited version of that scene to remove the mention of Derpy's name, to change her voice, and to correct her cross-eyed appearance.
Despite the fallout stemming from her first- and likely last- scene, Derpy Hooves has since been fully embraced by companies contracted by Hasbro to produce merchandise for the show. Far from being erased from history, the grey pegasus lives on in trading cards, vinyl figures, and clothing. She even made several (tiny) cameos in the finale of season three. News of her death, it appears, has been greatly exaggerated.
Be sure to catch The Last Rounderp, in its unedited original form, after the break.