A spokesman from the government station said the producers had not sought permission from the editorial committee to add the footage to the end of the episode.
He said the footage did not have anything to do with Legco, or issues discussed in the programme.
The management would investigate the alleged violation, he said, adding the extra footage will be deleted before the episode in question was uploaded for archive.
The spokesman stressed the 1989 Tiananmen Square crackdown or certain political topics aren’t banned, saying the run held ahead of the anniversary of the crackdown was reported by the daily news team.
RTHK has pulled several programmes after the new director of broadcasting, Patrick Li, was said to have considered them to be biased or inaccurate.
The station has also been deleting shows from social media, which critics say would allow the authorities to whitewash history.