A HUGE setback in any injury accident case, is not being absolutely honest with your doctors and the authorities. Your credibility as a victim is extremely important. Oftentimes an incident or claim can only be proven based on what the victim says and what the vehicle driver says. Thus, if you give statements to your doctors or the authorities that turn out to be false, this will likely destroy it completely.
Although it sounds like a cliche, honesty is absolutely the best policy. Be above reproach when it comes to describing what happened in the incident and how the injuries have affected you. Even minor exaggerations or incomplete versions of the incident you tell to the doctor can cause major problems in the claim. Also, be aware that statements you make to others can be used against you. If you make a false statement to someone like a witness or co-worker or friend, that person can be used as a witness against you. So being honest to everyone is the best policy.