Later that day, a longer video - eight minutes, this time - of the same event appeared on YouTube. In the week that followed, more clips were posted, which showed a more complete picture of the controversial incident.
It turned out that Lam was trying to speak out against the police's unfair response during a protest. Her case was a clear illustration of the importance of how citizen journalism and social media could help uncover the truth.
Something similar happened during the chief executive's public forums in August, when a boy claimed in front of several members of the press that he had been beaten up after shouting slogans supporting Leung Chun-ying.
Within a few hours, observant bloggers and social-network users found videos online which showed the same boy throwing a bottle at protesters.
With the rise of new media and online news blogs and websites, the number of local citizen journalists is growing, too.
Tens of thousands of videos, photos and articles on what is happening around the city are being uploaded to websites and forums daily.
Even some mainstream media now search for news stories through these emerging sources.
Citizens and social media play a very important role today in influencing opinions and revealing the complete truth about controversial events.
Thanks to citizen journalists using social media, cover-ups during the Malaysian general election, demonstrations in Istanbul, and the Syrian civil war have been exposed and have received global attention.
What's more, the mainstream media has been condemned for providing biased reporting. Citizen journalism allows close monitoring by people from all walks of life, and therefore a greater likelihood for truth.
All this is possible only if there is a free internet. In a society where internet censorship prevails, citizen journalism is hard to practise, and social media may be in danger of being used as propaganda by the authorities - like on the mainland.
Protecting the free flow of information in the online would, therefore, is the first and foremost step for ensuring the truth prevails.