1. Employee creates side project
2. Employee quits to pursue side project as a company
3. Former employer decides to sue employee because they don't like it for whatever reason (this need not have any legitimate chance of winning in court, it could be entirely frivolous)
4. Former employee then has to decide between spending hundreds of thousands of dollars in legal fees fighting a lawsuit from the former employer or simply giving up
We saw this just the other day on HN. I've seen it happen to friends and colleagues.
Best thing to do: no side projects. If you have those ambitions, work as a contractor. The legal agreements are much more rational. You get paid for the hours you bill and are allowed to do any work you want outside of your billable time.