Imagine for a second, if you can, that you are an independent contractor at a major corporation.
You have your role. You signed your contract, and you know exactly what you'll be doing each and every day when you arrive at the office. You have a deadline, and it's a couple of days away, but everything is going swimmingly. You're all set to complete your tasks and fulfill your contract. Everything is awesome.
Only, a few days before your deadline, your boss comes up to you. He tells you that another guy on your team was involved in a car wreck, and he won't be able to work for a while.