State of Arizona Substitute W-9 and ACH Vendor This form is required in order to establish or update a vendor account with the S-3 vendor account. Filing Errors & Tax Filing Mistakes Tax Filing Mistakes are a real problem! Every year, it seems like there are a lot of them. It takes a lot of time and money to correct them, and, of course, the problem gets worse. There is really nothing to prevent you from making a mistake, and with enough money, you could certainly pull it off! There are some common mistakes that we frequently see. And there's a good chance that if you get to it early, they can be corrected easily. So… here's some helpful hints for filing a Form W-9 correctly and efficiently. Mistake #1: Filing Tax Returns With a W-9 If you're going to do it yourself, you're going to have to take some risk. Your chances of getting caught up in a penalty-and-fines debacle are quite good. Even if you file a tax return with a W-9 (including claiming a refund), you will want to take steps to protect yourself and your business from criminal charges. Don't forget, though: a W-9 form shows someone knows who you are. You don't want to get a call from the IRS asking about “some old dude at a bar” who paid people off! The first and most important step of your W-9 filing is to go to the web address: and follow the instructions found there. Don't be surprised if it takes you over an hour to get through all the steps, especially if you're filing a big complex one! You can use this time to go through the Frequently Asked Questions, too. You never know what answers you'll find to these questions, when you're working to prepare a tax return. At some point, though, you will need to know your total for the year, too—your gross income or total compensation. Mistake #2: Filing W-8s & W-9s There are two major forms that you need to file using the W-9. First, the Form W-8 : This is a form you filed with your employer. You don't need this form anymore.