It depends. For me so far the problems have been easily sorted out except once. The once was a lot of extra and unnecessary calculations but essentially no pen

It depends. For me so far the problems have been easily sorted out except once. The once was a lot of extra and unnecessary calculations but essentially no penalty. Just the cost of my time, which is bad enough. My main complaint (which is still fairly minor) is that they still too often presume that "it" (the reason why they want more money) is the fault of the individual taxpayer. No need for them to consider other possibilities. Once, a client provided poor numbers on their 1099 and I used my version. Both versions based on my invoices. IRS doesn't bother to ask for a clarification. Just sent me a bill and fine. And I suspect in the end after my response letter, they audited the client. Not the best result but only a little stressful on my side.