From a civil standpoint I've settled cases in the middle of trial and afterwards had serious questions of whether or not I should have continued on and let a jury decide. Had a few of those that kept me awake.

From a criminal standpoint-and this is far more serious (obviously)-I've lost a case or two knowing in my heart that the prosecution either falsified the evidence or did something equally nefarious. One of my x partners lost a homicide case back in the 70s and actually witnesses the execution. That was the last criminal case he ever wanted to do because of the lasting effects.