Sealing door hinges before final fitment. This is a classic mistake that most take no notice off. You must use a chalking compound around the hinges if not you will create water leaks into the pillars. They will then over flow and capillary action will take place creating rust in floor pans etc. It will also create humidity which will dampen the whole inertia of the car.
A wet car is a rusty car.
GM always sealed under the hinges however they did not paint under them. The hinges were fitted when the car was in bare metal.