My partner and I were on the defense. Our mock trial client was charged with murder and we believed we had a clever defense "angle" on the case. Our theory was our client could not have shot the victim as alleged because he'd been shot in a rocking chair and the bullet trajectory was inconsistent with the prosecution's key witness. During our closing argument we used a rocking chair as demonstrative evidence.
My mistake? Not putting it away after I was finished. I left the chair in the middle of the courtroom as the prosecutor stood up to give his rebuttal. Needless to say, my opponent used the chair to shred my defense in the most embarrassing way. Even my wife cringed as the rocking chair was shoved down my figurative throat. Had I simply put the exhibit away I think I would have avoided the shock. That was the lesson.
In my work as a criminal defense attorney I often use various exhibits and presentations to illustrate evidence to the jury. I will have a witness introduce the exhibit and then describe how the information presented is relevant to our case. After I've gotten what I want, I put the exhibit away. Leaving something out not only looks tacky and unorganized, it gives my opponent (and the jury) a chance to deconstruct my argument and use my exhibit against me.
So that's Rule #17. When you're finished with it, put the darn thing away. Don't let the proverbial Rocking Chair get between you and a victory.