A key milestone would be the first occasion of tear gassing of peaceful protestors in decades, which at the time was directed by the chief executive C Y Leung, and this  sparked backlash from the public and precipitated the Umbrella Movement.
A pivotal moment would be the first instance of tear gassing of non-violent demonstrators in years, which at that time was ordered by top official C Y Leung, and this led to widespread criticism from the public and triggered the Umbrella Movement.