I think one could be in a problem if the illegal work (recorded in W2) was done on recent past dating back to 2-3 years. But even then you are most likely safe if you have not done it for 180 days or more. My personal feeling is that USCIS doesnt give much of a fuss about this as long as it is not done very recently though the rule says otherwise. I am yet to see or hear about anyone who was denied because of this reason.