What was the biggest procedural error made at Socrates' Trial that no fair legal system today would make? Why didn't those Greeks think about it back then?

The Athenians followed some sound procedures in their legal cases that we respect today, like having the jury decide one's guilt or innocence and letting the defense speak last. We know a fair amount about how courtroom speeches worked, not to be sure in Socrates' own time but in the decades after his death, because we have over 100 speeches by plaintiffs and defendants. Plato tells us a lot about the trial of Socrates but we can't trust him entirely. After all his version differs from that of Xenophon. So anything we say will have to be provisional and attentive to the limitations on our evidence. Some of the weaknesses of ancient trials followed from the difference in technology between then and now, which translated into a difference in credentials and proof. John Dillon's book SALT AND OLIVES spends a lot of time discussing ancient legal cases, and sometimes points out how things we consider trivially easy had to be argued in court then. For instance, an ID card and birth certificate today...