In the second season episode "The Last Roundup", Derpy has her first real dialogue. In the scene in question, Rainbow Dash witnessed Derpy's goofiness for the first time. On one hand, the entire scene is incredibly hilarious. But then again, this scene attracted the attention of a group of parents much like the PTC (Parents Television Council); their complaint was that the character of Derpy ridicules of the mentally disabled. As absurd as this accusation is, Hasbro simply panicked and altered the scene. A detailed comparison between both versions with pictures can be found here.
According to letter/post from Lauren Faust, the opening scenes were supposed to be/ or contain Rainbow Dash and Derpy jumping on the rain clouds, causing the lightning strikes, and Derpy gets a little carried away. This was in the original airing, but had this been in later airings (and subsequent Netflix streaming), it would have made the scene after the opening credits make more sense. Another change was that, instead of being referred to as "Derpy" as she originally was, she is now called "Ditzy Doo". It was also mentioned in the letter that for just about every parent that complained, about twenty praised the show for finally introducing Derpy as a full-fledged character.
Sadly, it has been rumored and possibly confirmed, that due to these close-minded parents who complained about poor Derpy Hooves, she will no longer receive any dialogue parts, but she still has the potential to make quick background appearances in episodes, as she did in the third season finale, "Magical Mystery Cure".
This has been proven untrue though, as Derpy receives several lines of dialogue in Season 5, episode 9, Slice of life. HEr name however has been changed to Muffin.