It seemed valid at the time, but the rumor that Derpy Hooves/Muffins (however you came to know her) would never appear in the show again after Season 2.
In an old pony RP I was part of, Rarity said that her parents died in a fire.
About a week later, Rarity’s parents appeared in the show. The episode began with Rarity waking up to a fire Sweetie Belle had caused. They lived, so far as I know.
I think nothing can beat that whole “Discord is Starswirl” rumor. Someone even “leaked” the supposed dialogue from some future episode all the way back in 2012, but it turned out to be completely made up. Ironically, Discord did turn out to be Grogar in the finale, so the idea of Discord pretending to be someone else was not that far off after all.