Based on the information given, Sam made a mistake oh having a positive prejudice about the speaker.
Prejudice simply means a preconceived opinion about a particular person or group that's different from the actual experience.
In this case, Sam left the venue and posted on social media about how great the speaker was and then found out from a friend that the second half of the speech went against everything Sam believed in.
This illustrated that Sam made a mistake of having a positive prejudice about the speaker.
Learn more about prejudice on:
https://brainly.com/question/25189630