Hundreds of council workers in Blackpool were exposed to pornography during a conference presentation.
An obscene image flashed up twice as a video about the seaside resort was played to delegates at the town's Winter Gardens.
The Labour-run council has apologised for the blunder, and said the audio-visual arrangements were handled by an external contractor.
Opposition leader Tony Williams said the "upsetting" error was embarrassing.
"The audio visual arrangements for the conference were handled by an external company using their own laptop," a council spokesperson told the Local Democracy Reporting Service.
"We have asked them to carry out a thorough investigation as to how this happened."
The spokesperson added: "A full apology was made to delegates during the conference for any offence caused."
Councillor Williams, leader of the Conservative opposition group, said many people in the audience had been upset by what they saw and branded it "embarrassing".
Mr Williams, who was not at the conference, said: "For some of the people there, this was very upsetting.
"We need assurances this won't happen again and there needs to be a full, independent investigation into how it happened."