I was being a little bit glib there. The judge did not literally ask me if I was guilty and I did not literally take the fifth. What she did was ask whether the thing the cop claimed to have seen, which he could not possibly have seen, nonetheless actually happened, and I said I could not remember because it had happened more than a year ago.

It was a very weird situation from a moral point of view because on the one hand, finding me guilty was not supported by the evidence, but on the other hand, letting me get off scot-free would have been even more unjust because the fact of the matter is that I did commit a fairly serious moving violation, and the only reason we were in court at all was that the cop decided to cut be a break and then took a long time to do the paperwork.

You said you had photographic evidence...were you not allowed to introduce it? Why does it matter if you can't remember if you have video proof?

I had proof that the cop could not have seen what he claimed to have seen, and that therefore he must be lying about having seen it. I did not have proof that the thing he claimed to have seen did not actually happen.

Yes...I'm confused about OPs story because, as far as I know, you can't just plead the 5th at various parts of a trial where it is not relevant. The judge would generally not move on until you plead something. Or they would hold you in contempt if you refuse to plead guilty/not guilty/alford/nolo contendere etc

Well, it's a magistrate. They tend to not know/follow the laws/rules in my experience. They aren't even required to have a law degree or even pass the Bar. I think in my state they only require 40 hours of training.

Based on their other comment it sounds like they said they didn't remember when asked by the magistrate if they committed the act, which isn't the same as invoking the fifth. Thwy also said they were able to prove the officer couldn't have witnessed the act they were testifying about. Then the magistrate found them guilty (based on false testimony?).