I think their biggest error was allowing both Skilling and Lay to testify.
Both lost their cool on the stand.
It was a calculated move, I suppose. The defense probably thought they were going to lose and perhaps a stellar performance on the stand could save their bacon.
Both defendants actually did pretty well under direct examination by the defense at first. But neither one could stand up to cross-examination by the prosecution. They came across as bitter jerks, not something the jury could show much sympathy for.
The fact of the matter is that neither defendant had much chance of securing a not guilty verdict. Houston juries tend to be extremely tough, and this city was hurt deeply by the Enron collapse. Lay's and Skilling's only hope was a change of venue to a place where they could get an OJ jury. When that motion was denied, it was pretty much a foregone conclusion that both would serve hard time.
Ken Lay is 64 years old. His prison sentence has a strong possibility of being a life sentence in practical terms. Skilling is much younger, but he'll be released as an old man. Together they spent over $38 million in their joint defense. To say they are ruined is an understatement.