I have ran into a few occurrences where fabrication units that are given a queue of buildings to build will sometimes continually run into an obstruction instead of finding a path around it. I have also ran into issues where in some cases units can be commanded into orbit around a planet but get stuck into orbit around the star instead if the planet is in a close orbit to the star. It is also possible to assist in building enemy structures(bug?).
Ok... This isn't actually the AI. It's a pathing issue. Pathing is still in the works. This is a known issue as far as I know. And yeah. Assisting in building enemy structures is fun. Play equilibrium with the opponents commander - half of your fabbers reclaim him. The other half repair him.