My number one is buy hair color, then use said hair color.
Did you know when you're tired that the words "Lightest Golden Brown" can look like "LIghtest Golden Blonde"? Especially when the company uses the same model on the bos.
Lesson learned: Brown is not the same as blonde and READ THE COLOR NAME! Off to the store for the right color. Hey, but I still have 4 days until the wedding, no worries, right?