Perpetual motion!
Quite a lot of good material here and at the links it contains.
In general: you don't get something for nothing. Or as science fiction fans like to say "There ain't no such thing as a free lunch." or TANSTAAFL.
Every now and then somebody comes up with what seems like an extra increment of work from a system. This can usually be explained by sloppiness in the design.
Sometimes there is out-and-out fraud - a hidden battery, footpedal, mouse (the furry kind)...
