Earlier quoted context omitted.
FWIW, some folks may have employment contracts with $dayjob which give that company intellectual property claims on all technical work by employees whether done on or off the job. The extent of those claims obviously depends on the contract itself, and enforceability varies by jurisdiction. (California is better on this than most US States, but even there, employers with a will may have enough wiggle room to make tro…
About this type of clauses, I've heard that "well, they are standard and are almost always ignored, as long as you don't cross the line (e.g. code for your side-projects in your day-job office)". I know this depends on the company, but, as a general rule, are these clauses really taken so lightly?
In short, by taking these clauses lightly, you are creating failure paths for yourself that begin from the initial light of success. You may not want to kill your big success even before it happens. If your goal is just to create a side income, you would most likely be fine simply because a lawsuit would cost more money than what it may recover, or you may just get some threats/scoldings.
I have heard that if your side project or startup reaches something like $10 million in revenues, your big-company employer actually becomes liable to their shareholders to recover this money from you since it technically belongs to the shareholders.
Some high-profile entrepreneurs have told me that you should not worry about lawsuits as they only come when you succeed and when you do, you have resources to take care of them. But I have also known specific cases of things doing down because of such issues (like the example from etsimm, who also responded to your comment).
By the way, YCombinator submission form includes a question on such IP issues, though I may guess everyone just blindly picks "No issues" for it.