In the Netherlands the law is pretty straigtforward that this is a bad idea: > The "Nature of Employment" Rule: If you are hired as a software developer, almost any software you create (even in your own time) can be claimed by your employer.

We always advise our employees to request an exception for it. We are pretty relaxed about it, but we don't give out a blanket exception

Has that law ever been enforced ? (e.g. taking away a FOSS or other project that someone wrote in their own time)

Luckily in California your own time is your own time.