I can think of a few things, all involving cameras. A branch over the speed limit sign the most odvious...leaves deflecting the radar beam...some other reason it points in the wrong direction...All bs and some of it contrived, but still it requires resources to prove against and what's to lose? A little time? I'ld give it up along with a slightly higher fine just to be a pain.
In that previous post where I said the State failed on every single seat belt violation — OTOH, every single speeding ticket was found to have been committed, except for one where the trooper mistakenly wrote a speed on the ticket which was higher than what the radar showed. The judges have heard it all. This particular judge was actually quite knowledgeable on the technicals of the data capture systems that were used. Great entertainment. But I do think citizens should defend themselves within reason against charges — otherwise the state becomes a tyranny.