So, a reactionless drive (that is, a motor that produces a net change in global momentum other than 0) violates conservation of energy in some reference frame. This is a known result. (Project Rho has a section covering this)
But what exactly do we have to do to (or can we even) make it violate conservation of energy in its own reference frame? That is, given a complex device D initially at rest, with no initially moving parts, in a total void, how simple can we make D such that in D's reference frame there is a violation of the conservation of energy?
The basic idea, is that if you're going to go ahead and allow the existence of a reactionless drive in your universe, you might as well actually take advantage of it as an unlimited power source as well, if possible locally. Such as a power station on a planet, or a reactor core in a space craft.
All Shadow priest spells that deal Fire damage now appear green.
Big freaky cereal boxes of death.