Originally Posted by FetishGeek

It is illegal, and there are penalties for filing a false DMCA request. If you receive a DMCA request you believe to be false, you could file a lawsuit against that person. If, in the case of the lawsuit, it is revealed that they knowingly filed a false take down notice, you could receive a serious financial reward.

However, if you can be classified as a service provider, you can not immediately be sued by the copyright infringement of your users. If you receive a take down notice, and ignore it, you loose that protection. You now can be sued. So the danger, if you ignore the take down notice, is that you now have legal liability for that infringement, if it can be proved that infringement did occur.

Another possibility, if you ignore the take down notice, they could send the same notice to the service providers you rely on to keep your site online- Your ISP, your hosting company, your payment processing company, the networks that provide advertising on your site. This now notifies them that your site hosts pirated content, if they continue to provide services to you, they could be legally liable for your infringement. Often, in these cases, these companies are going to opt to drop you as a customer instead of take on that kind of liability.

Typically, copyright is assigned the moment a work is created to its creator. It does not need to be registered, though registration can help prove ownership. Unless you are creating your content yourself, or licensing it, or the content already comes with some sort of license allowing you to use it, you can't just use it. It belongs to someone and if you just start using it without permission, you are committing copyright infringement. In the same way, any content you create is yours. Someone else can't just come along and help themselves to your content.