There are a lot of situations where my lawyer's mistake might cause me to go to jail for a crime I didn't commit. There could be some prejudicial evidence that he failed to get excluded, or exculpatory evidence that he failed to get included, or an incorrect ruling in the trial that wasn't properly preserved for appeal.

> In which case would this be the primary blame, and not the legal system itself, or you for actually doing the crime?

There are plenty of ways a lawyer can screw up procedurally that can drastically impact your case.

One of the nice quirks of USA justice system is that you don't have to be guilty of a crime to go to prison, just that the prosecution has to convince 12 people beyond reasonable doubt that you are guilty.

The sixth is taken seriously to the point that incompetent lawyer could be a valid reason for granting a retrial.