Ask HN: Has your employer ever claimed ownership over a personal project?

I understand according to many (most?) employer contracts, anything you develop or invent at company offices -or- during work hours -or- with company equipment becomes the property of the employer. Regardless, I often hear about people working on personal projects during down time at work. While I have no desire to do anything like this, I always wonder how they get away with it, or _if_ they even do.