Depends on whether it is in your favor or not. If it is in your favor, pay it and don't say anything. It could very well be that the officer was trying to do you a favor and lower the fine. Usually they write a lower speed on your part. If it was truly an error, he wrote 25mph zone instead of 35mph zone, I'd contest it, there is more money and possibly more points in a larger difference. And document the issue carefully. The speed zones are well known and to show the error should be pretty obvious.