I had a side project that made some money and was eventually sold. Of course it became an official business entity much after I started working on it. I don't remember the exact timing.

At a later time, when I was applying for a job at a big company, I was informed by HR there was a mistake on my resume. They said I had the wrong start date on my one man company! I assume they looked at the incorporation date and then thought I was fudging my resume.