After many months, your software sale is complete! You've got a purchase order, sent the invoice, delivered the software. You're already handling some support issues from users at BigCorp. Then BANG! Martha from Procurement emails back, as a favour, just to let you know that BigCorp has not received your W8 form with a valid tax id, and therefore will be withholding 30% of the purchase price of your multi-thousand dollar product for taxes.