"I think I made a mistake once... yeah... it was only once"

In this quote, Jay Mohr is reviewing a past mistake that he made. The repetition of "once" emphasizes the reality that it was a particular event, suggesting that he is not one to make errors often. Using "yeah" includes a casual tone to the declaration, as if he is practically shrugging it off. This might imply that the error was not a major one, or that he has actually carried on from it. In general, the quote communicates a sense of self-awareness and approval of one's flaws, in addition to a tip of humor.