Fluttershy has a nightmare that Discord hated her.

Fluttershy thought the dream was real, so she gives Discord the silent treatment the next day. Discord feels so bad that he gives Fluttershy the silent treatment, too. Fluttershy and Discord are now officially ex-best friends.

It isn't until Discord rescues Fluttershy that Fluttershy realizes that Discord is still her best friend and that dreams aren't real.