An article at NasaSpaceFlight.com is claiming that the superficially reactionless EmDrive has again been tested at NASA Eagleworks, this time in hard vacuum, and the anomalous thrust is still being detected:
A group at NASA's Johnson Space Center has successfully tested an electromagnetic (EM) propulsion drive in a vacuum – a major breakthrough for a multi-year international effort comprising several competing research teams. Thrust measurements of the EM Drive defy classical physics' expectations that such a closed (microwave) cavity should be unusable for space propulsion because of the law of conservation of momentum.
With the popular explanations of thermal convection or atmospheric ionization being ruled out by operation in vacuum, and thrust thousands of times greater than expected from a photon rocket, is it time to start taking the EM Drive seriously as a fundamentally new form of propulsion, and possibly a door to new physics?
Roger Shawyer, the inventor of the EmDrive, claims that the device's efficiency will scale even further with greater levels of power, potentially enabling fast interstellar travel powered by a radioisotope thermoelectric generator or nuclear fission.
Previously: NASA Validates "Impossible" Space Drive's Thrust
Yes, it's called a Brownian ratchet [wikipedia.org] and, although it took them a long time to do so, scientists finally figured out why it wouldn't work.
The PM Wiki article is quite in-depth. It's a good read.
The other "gotcha" they mention is that things that *appear* to be perpetual motion machines (e.g. something that gets its input from tidal energy or radioactive decay) may work for an extremely long amount of time, but they will eventually fail because of entropy (plutonium will eventually be totally decayed in millions of years).