Just as Tempest is lunging at Twilight, preparing to hit her with the stone that would freeze her in crystal…

Look to the left of the screen. That’s Derpy, rushing in to take the bullet for her just as Rainbow Dash picks Twilight up and carries her away.

The reveal that it was Derpy who was frozen in the crystal was not a coincidence or just meant for comedic effect. Derpy sacrificed herself for Princess Twilight, which in my opinion was the most heroic thing that happened in this movie.