US patent number 2886976 is a patent that seems to fail the "usability" criteria for patentability in the USA. Note: this is not a software patent.
 System for Converting Rotary Motion into Unidirectional Motion
The invention relates to driving systems for producing unidirectional motion and it has for its primary object to provide a propulsion system in which a rotational movement produced by a prime mover is converted into a continuous or intermittent unidirectional movement of a load carrier which may or may not be the carrier of the rotational elements of the system.
The US Patent Office requires that the device be "useful". This device cannot work as described, it is a violation of Newton's Third Law of Motion which states "To every action there is an equal and opposite reaction." The inventor, Norman Lormer Dean, believed that Newton had missed a time element, i.e. that there was a delay between the action and the reaction. This is unsupported by scientific theory. There are a variety of web sites devoted to this patent, both for and against.
- Dean Space Drive site - Dedicated to the work of Norman Lormer Dean
- Wikipedia Entry on the Dean Drive
- Wikipedia entry on Reactionless Drives
- Jerry Pournelle's story of his involvement with the Dean Drive
 Final Note
This patent, which is for what is commonly called the Dean Drive may be the most notorious patent ever issued by the US Patent Office. It was never legally overturned, despite the scientific impossibility of it's functioning. The arguments made by Norman Dean convinced many people that it actually worked, including Science Fiction Author and Editor John W. Campbell. The reason Dean was able to convince people that it worked, was that when the device was on a surface, that the device, which contained rotating weights, moved. The movement was actually caused by friction between the casing and the surface, as the weights moved, the device moved, and the friction prevented the device from moving back as the weights moved in the other direction. Tests with devices like this which were conducted in vacuum, where friction was not an issue, showed no movement.
Interestingly enough, this patent has been cited numerous times. One invention which cited this patent as prior art, was a rotary concrete breaker. What Norman Dean thought of his Space Drive being cited as prior art for a concrete is not known, he died in 1972 before that patent was filed.