Early on, McConaughey realizes that with the deep-rooted racist sentiment in Mississippi, there's no way in hell a black man will get a fair trial, even if that black man is Samuel L. Jackson. Not wanting to "pull a Finch," he wisely submits a motion to change the venue of the trial, which the judge, who is being pressured by the KKK, summarily denies. Distraught about this, Matt mopes for a while before Sandra Bullock (his spunky, ambitious legal assistant) points out that the judge overlooked a legal precedent in the change of venue motion. Armed with this new information, McConaughey resubmits his motion ... and has it denied again.
"Damnit. I thought I had all the law. He just lawyered the pants off of me."
Now, the first time the judge denied McConaughey's request to move the trial (without checking the legal precedents), it was a screw-up on the part of the judge. When a judge fails to consider all the evidence in a legal motion, this is called a "reversible error" (which, in legalese, means the judge done crapped him the bed something fierce). Bad enough, in fact, for the whole show to be declared a mistrial, meaning the defendant gets a brand-spanking-new trial. McConaughey has two options: 1) tell the judge that he made a mistake and give him a chance to correct it or 2) don't tell the judge; keep the information a secret and hold onto it. McConaughey chose option 1. He informed the judge of the screw-up and gave the judge a new opportunity to review the information. And he still got rejected.
Here's what McConaughey should have done: nothing. When you see a judge has committed a reversible error, you shut the hell up. That's the ace you hold up your sleeve, the one you don't play until the last minute. McConaughey should have kept quiet about the reversible error until after the trial, because a judge making a reversible error is grounds for a mistrial. Let's say Samuel L. Jackson lost the trial -- that's when McConaughey should play the "reversible error" card and get a brand new judge, jury and trial. Holding onto a reversible error is like having a mulligan, or a Get-Out-of-Death-Penalty-Free card. He could have just waited for the outcome of the trial and then said, "Nah, I don't like that. Do over."
"Only this time, we all wear party hats."