Have you ever been sued?
After you finish your work on the sign, and a piece falls off and hurts someone, and they go after your customer, his lawyer will be giving you a call, get ready, you might want to get a lawyer on retainer and be ready for the party... Oh, like others have said, have copies of your professional certifications and permits approved by the city ready to go!
Later, you can look back and say to yourself,
"you know it would have been better, if I would have taken the advice of the folks on Signs 101 and saved all this money I'm giving my lawyer".