16. (14 points) Player A and Player B are having a table tennis match, using a best-of-five format (i.e., the first to win three games wins the championship). For each game, the probability of Player A winning is $\frac{2}{3}$, and the probability of Player B winning is $\frac{1}{3}$. If the event of "Player B winning the championship" is referred to as "an upset," find the probability of this event occurring.
16. If one side wins with a score of 3:1 or 3:0, the remaining game is played but does not affect the match result. Therefore, the problem is transformed into: finding the probability that "Party B wins at least three out of five games."
The probability of Party B winning all five games is $\left(\frac{1}{3}\right)^{5}$;
The probability of Party B winning four games and losing one is $\mathrm{C}_{5}^{1}\left(\frac{1}{3}\right)^{4} \times \frac{2}{3}$;
The probability of Party B winning two games and losing two is $\mathrm{C}_{5}^{2}\left(\frac{1}{3}\right)^{3} \times\left(\frac{2}{3}\right)^{2}$.
Adding the above results, the probability that Party B wins at least three out of five games is $\frac{17}{81}$.