There are two exceptions listed on 2870, the first one is going to be the gotcha. It excludes inventions that:
> (1)Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer;
So, if you work at $BIGCO, they will argue that since they have their fingers in everything, that anything you might work on "relates" to their business or actual or demonstrably anticipated R&D. This is a truck-sized loophole.
> (1)Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer;
So, if you work at $BIGCO, they will argue that since they have their fingers in everything, that anything you might work on "relates" to their business or actual or demonstrably anticipated R&D. This is a truck-sized loophole.