A friend of mine who had a 427 Cobra, back in the day, got nailed by one of those stations for loud exhaust. The fix-it ticket needed a way to get fixed without having to actually change the exhaust system at all.
So, he went and got his girl friend's new-ish car and went back on a different day to the pop-up check station. The new car didn't have any trouble getting a passing sticker, so when he got home, he heated up and pulled off the sticker and put it on the windshield of his Cobra.
Somehow, that sticker worked its magic and some officer signed off on the ticket.
After that, the Cobra got waved past any CHP check station...so long as my friend had the clutch in and the revs down to an idle.