Nexus is not an issue for someone with physical presence.  In the context of recent court decisions,  It's an on/off switch for those with "contacts" that are based on factors other than being there.  Some states have laws that exempt individuals with nexus from paying tax, if the amount of income or the length of stay is below certain standards.

If you formerly practiced in California, you know the FTB sometimes takes a while to get around to conducting an audit.  Let's say her 4 months in SF generated the Ohio 1099.   Let's say that company is in Cleveland, and Sacramento sends an auditor there only twice a year.  When they show up in September, the company's penalty for not filing the correct 1099 will be reduced if they get around to doing it right away.  Or maybe the Ohio company has an accountant that hasn't dealt with California filing requirements until this year, but attends a seminar next month and learns what should have been filed.  

California, like IRS, doesn't tax pieces of paper.  It taxes income.  Your mindset should not be "no piece of paper, no tax ."  It should be, "let's pay the right amount of tax to the right state."