Ddvols gives an example of a scenario that I've seen several times over the years (W-2 secretary during the day; 1099 occasional cleaning person at night). That's perfectly legit.

Form 8919 coupled with Form SS-8 would be the recourse if the employer has been playing fast and loose with the employer/employee relationship. Three considerations: (1) As someone pointed out, it had better be an ex-employer because filing those forms essentially puts the employer on report with the IRS. (2) Make sure the client knows that if he wins, once all the dust settles he will save only about 7.65% of the amount (or "profit") on the 1099, so he should consider carefully the pro's and con's of lighting the SS-8/Form 8919 fuse. (3) Having lit the fuse, there is no guarantee that the IRS will ultimately side with the employee over the employer, although admittedly the IRS goes into the decision leaning towards an employer/employee outcome versus an independent contractor result. I've handled only a handful of these cases, but in every case, the IRS ruled in favor of the employee.

Finally, I didn't follow Shorebird's "take off the expenses subject to 2%" comment. I might be missing something here.