As intelligent and good-looking as we are, there are still some times when we need to be reminded to keep our fingers off the hot stove, don't play in traffic, and don't mix beer and whiskey  Oh, wait, scratch that last one.  Anyway, there are some automotive things that should never be done.  Some are performance, some are cosmetic.  At any rate, pay attention.

1.  Truck bed tailgates -- up or down?  This is a trick question, because until the scientific test results are known, you'd think it would make more sense to put the tailgate down to reduce wind drag and get better gas mileage.  Surprising to say, it's better to leave the gate up.  Here's why -- when the tailgate is up, the air in the truck bed is pretty much stationary and acts like a solid object, allowing the wind to flow over it, almost like one of those fancy truck bed covers.  When the tailgate is down, the airflow is now scraping along the entire inside of the bed, creating more drag and turbulence than you thought possible, and this robs you of gas mileage.  The only thing worse than having the tailgate down is to remove the tailgate and put one of those net things where the tailgate was.  Now you have wind drag all in the bed, plus you have the drag-chute effect of the net working against you. 
  An episode of the TV show, "Mythbusters", covered this recently.  Quite convincing.
  So if you're on the highway, either get a bed cover or leave the tailgate up and fastened for the best gas mileage.  Unless you just really want to clean out the back of the truck.  ;-)

2.  Lowered chassis -- Looks good in a show or on flat dry streets, but for everyday driving they are murder on your chassis and your back.  If it rains, you're the first one to flood. 

3.  Hydraulic chassis -- Few things look stupider than a vehicle with its back bumper dragging the ground and the front pointing to the sky.  The cruisers in LA might do it for show, but it's no good for actual driving.  Save your money for the gas to drive it.

4.  Wheel spinners -- Soooooo lame!  They were interesting for five minutes after they came out, now they are just wack.  Remember those decals kids would put on their back glass that said "HOT"?  Yeah, it's that lame.

5. Lights under the chassis -- Why is anyone wasting money on this garbage?  Also sooooooo lame.

6.  Loud stereo -- Yeah, those thudding booming house-shaking rolling boomboxes.  The first mistake these drivers make is assuming that everyone within a half-mile radius wants to listen to the driver's favorite noise.  Got news for you pal -- nobody wants to hear it  but you.  So keep it down.  Would you like it if someone came by your house while you were trying to sleep and your whole house was shaking to some kind of awful music that you really really hate?  Not only is it detrimental to your hearing and health, in many cities it will get you a really big fine.  $250 or more, and it usually gets bigger with repeat offenses.  I've heard of stereos being ripped out by the judge's orders, or by vigilantes.  Save your money for something useful, like gas and car payments. 

7.  Wide tires -- tires should never stick out beyond the wheel wells.  Out there, the tires throw road junk on your paint job and on other drivers, it messes up the handling dynamics, and it just plain looks dumb.

8.  Dumbest wheel treatment ever -- one time I saw an 80s Suburban with what looked like ten-inch rims.  Unbelievable.  Imagine lawnmower wheels on a Suburban.  Stick with the stock size and you'll be safe.  Those 'twenties' and larger rims with the quarter-inch-thick tires are bad for the car chassis and they are getting to be like wheel spinners -- lame.  The fad's over, already.