Spencer pushes Mona off the cliff during their fight.
The episode centers on the Liars (Aria, Emily, Hanna, and Spencer) learning the true identity of the main tormentor, "A." It is revealed that Mona Vanderwaal is A. [S2E25] Unmasked
Mona tricks Spencer into going to a cliff-side "A's" lair (near a hotel area). Mona tries to force Spencer to join her "A Team," and when Spencer refuses, a struggle ensues. Spencer pushes Mona off the cliff during their fight