Here's my list
#1 - Not getting some one to look at the car prior to the show. Too many times IMHO owners finish up cars at the last moment or are hesitant to ask for help so they roll out their freshly finished cars to their first show for the car. This may be the owners/builders first or twentith car its all the same. No matter how good you are you always miss something and for some this misses can and often will cost you one or more award levels. We all miss a screw here or there, a bracket gets overlooked or a touch up and these misses mean little point deductions - again and again. Often the vast majority could be corrected in one full weekend for little money and its a shame IMHO that a car that other wise would do well instead does not. Please, please take advantage of the resources out there and put your best foot forward the first time out.
#2 - Over and over - DO NOT use black weather strip adhesive.
#3 - Clean the car, yes even those spots you don't think judges will see. Believe me, if you don' t they will park your car one the show field and the sun will be at the moment of judging shinning down illuminating that spot
#4 - Wrong/ borrowed paint marks. Suggestion if you add them remember to bring pictures of the marks from YOUR car with you in your handy little binder
"That's what I remember" - "So and so has them on his car" and similar claims will be looked at as suspect especially if we've seen the same ones on a car in the forums built at a different plant or year or just a different car just last month
Will reserve my 6 other choices for later ... need to get to work on another project.