OK, I should say, "it COULD happen," not it WILL happen. But I am aware of several cases.
The first is from my hometown, many years ago, when I lived in NC. Guy running a dry cleaning business paid his people as contractors and was eventually hammered for it. He lost the business for back taxes.
There are others: a neighbor of mine, a carpenter/handyman who hired people on the fly to help him, and didn't take out taxes on them. He ended up on a repayment plan to the IRS.
Radio stations, especially small town jobs, have been busted for this, too. I know of a couple of cases.
The most common scenario is that a disgruntled worker gets annoyed at how much he/she has to pay at tax time. They ask around -- maybe even call the IRS -- and are told that they were an employee, and that the person paying them should have withheld. In that case, the IRS goes after the business that had them working "on contract."
Yes, it DOES happen all the time. I've seen it. It may not happen right away, and yeah, it's possible that a really lucky businessman will never get caught. But it's the equivalent of speeding in heavy traffic, in a construction zone, with cops all over the place keeping an eye on things.