Harsher in Hindsight: GLaDOS's statements that everypony else thinks Ditzy/Derpy's retarded is lent support after the negative reactions to her appearance in The Last Roundup.
Moral Event Horizon: Acknowledged in-story. Chell has managed to leave Apeture behind her and is now living a happy, glorious life as a pony in Ponyville. When GLaDOS still manages to ruin her life and abduct all her friends, Chell decides that she's finally crossed the line.
What she does to Ditzy Doo is arguably even worse. Ditzy is able to deal with all of the verbal abuse aimed at her, but when GLaDOS starts mocking her for having a daughter, Dinky, and telling her that Dinky's probably inherited the conditions that make Ditzy stand apart from other ponies, that's when Ditzy's heart splits in half.
All of GLaDOS' interactions with the ponies are a quite interesting look at how much worse her usual taunts are when directed at someone other than Chell.