1. You must wear a shirt while driving.
It sounds silly, but its true. I know people who have gotten tickets for ignoring this law. Keep your shirt on while driving, even if the AC goes out.
2. If you run over a chicken, you must pay for it.
It is expected that if you kill someone's chicken, you will stop and find out who owns it. You will then pay for the chicken. Depending upon the age and size of the chicken and the attitude of the owner, it could cost you $5 - $15.
It is unlikely that you will be tracked down if you don't do this, but you should. For some people, that chicken may be the difference between eating or not.
I had forgotten to mention those items in my longer and more detailed post about driving in Panama. That post covers items like cell phone use, seat belts, insurance, and what type of driver's license you need to have.
In that post, I also discuss I what it is like to share the road with erratic taxis and more.
I hope you don't hit any chickens during your Panama travels.