In a case where the vote of 71 is unanimous to convict, they can't convict, but rather acquit - beginning with the fact that the sentencing could not be carried out on the same day of the vote to convict. Because nobody would come forward to argue against the conviction, which is essential in this kind of capital case. Plus, the people who are appointed to the Sanhedrin had to be of high stature, wisdom, look good, know 70 languages, and be exceptional at argumentation, among other factors. Also, delving into the 70-languages requirement - several towns who had these polyglots for the Sanhedrin. Plus, the Gemara names the people who are fit and sat on the Sanhedrin.