Copied from Ohr Somayach Ask the Rabbi http://ohr.edu/ask/ask131.htm
Question: You are one of the judges in a Jewish court in a capital case. It is your turn to state your opinion. If you say, “I find the defendant innocent,” then the defendant receives the death penalty. But if you say, “I find the defendant guilty,” he goes ‘scot-free.’ What is the case?
Answer: You are the last judge to vote, where all the other judges have already said ‘guilty.’
According to Torah law, convicting someone of a capital crime requires a Sanhedrin of 23 judges. After hearing testimony from eye-witnesses, the judges vote. If at least thirteen of the judges vote ‘guilty’ the defendant is executed.
There is a surprising exception to this, however if ALL the judges vote guilty, then the defendant is acquitted.
Here’s why:
There are two ways to look at everything. There’s no situation in this world without some merit or positive side. If not one judge was able to see the good side and declare the defendant innocent, something’s wrong. The positive side of the case must have been missing during the presentation of the evidence. Therefore, he is acquitted.