Even after a patent is issued for an invention the rights granted by the patent can be lost before the statutory 20 year period. Some of the most common ways in which patent rights may be lost include:

a. Failure to pay required maintenance fees;
b. Even though the PTO, through a patent examiner, has "approved" a patent application, the patent may be defeated if it can be proved that the patent does not show the best mode for making and using the invention, does not adequately describe the invention, or includes legally insufficient claims;
c. Prior art references not considered by the examiner or the inventor are found which show the invention wasn't actually novel or unobvious;
d. A patent may be defeated if it can be proved the inventor committed a "fraud" upon the PTO by intentionally misrepresenting facts in the application or failing to provide relevant material ( most often prior art references) to the PTO; and
e. Certain illegal uses of the patent by the owner including violation of antitrust laws may invalidate a patent.

In most cases such defects are found and used by an infringer to defend against an infringement suit, but often an inventor could have avoided such defects by using proper diligence and preparation.

Inventors and patent practitioners have a duty to disclose all relevant information material to a patent application to the PTO. Although it may be tempting to "hide" or "forget" a known prior art reference which may cause the patent to be rejected on novelty or unobviousness grounds, such information must be provided to the PTO

The patent laws also require a patent application to indicate the "best mode" for building and using the invention. On some occasions inventors have failed to disclose the best method of making an invention either out of fear that the invention will be stolen or in the hope that the best mode can be hidden for longer than the 20 year period of the patent. Patent rights may be defeated if such failure can be proved.