1. The officer doesn't show.
2. One officer shows, the other doesn't.
3. Somebody else shows trying to represent the officer.
4. You read the law and an element of the charge is missing.
5. Something the law requires for the offense to be charged is missing (certain signs for radar tickets in Montana, for example).
Not So Good Defenses (take another look at traffic school):
6. The officer made a mistake (ticketed wrong car, radar used improperly).
7. The police department did not supply material you subpoenaed.
8. Necessity (I had to speed to avoid an accident.)
Image courtesy of Tom Curtis