having you plate in your windshield now actually carries a larger fine than no plate at all. it is concidered a safety hazard because in a crash the plate could fly off the dash and cause you or your passenger bodily injury.
modified exhaust is a popular one, my brother just got that 3 weeks ago. clear corners and clear tails was concidered illegal vehicle equiptment due to the lack of the required red reflectors, or ambers in the front. tint is concidered illegal vehicle equiptment, according to wsp, my truck is 3%. modified ride height or vehicle to low, any part of your vehicle that hangs lower then the bottom of your rim is liable to get you a citation. even factory mud flaps.
any head light color other than standard white is illegal, this is harder to hand out citations for concidering most newer cars come with factory HID units that are not "standard" white bulbs.
every vehicle must have 2 tail/brake lights. which, incedently has a catch to it.. i had an 86 accord 4 door. which had a factory 5 brake lights. i got pulled over because one, just one of the 5 bulbs was out, and got a ticket. because it came with 5, it is the law it always have 5.
and one law most commonly over looked by hatchback owners, like my self, is when you remove your factory wing, you also remove your 3rd brake light, usually with a duckbill spoiler, that is, i want to say federal law but i cant be 100% on that. i know 3rd brake lights ARE required in washington.