basically anything you have added on after the car rolled off the showroom can be a defect. ok, maybe the christmas tree or dice hanging off your mirror (eeew) isn't but if a copper wants to be an idiot he can say they destract you therefore...
maybe that was a bad example but they can and will get you for most things if they want. it's their lack of knowledge that affects their judgement too. eg, i've heard of cops trying to defect a car for a stock bov. it's ridiculous right, but with limited knowledge they don't know if it's a stock item or not. they just defect you cause it's a 'BOV' and they know it's illegal.
btw, i'm getting an eng cert very soon but even that can't save you in some instances.