We’ve all been there. A client walks into your office and, somewhere in the conversation, you realize that a seemingly minor oversight, a missed deadline, a form nobody filed, an election nobody mentioned, has spiraled into a five- or six-figure tax problem. In my years of practice, some of the most expensive mistakes I’ve seen weren’t the result of aggressive planning gone wrong. They were small, quiet errors. The kind that happens when a deadline slips, an election isn’t made, or a form gets overlooked entirely. The tax code is unforgiving in these situations, and the IRS has little sympathy for “I didn’t know.” This article walks through some of the most common, and most costly, small mistakes that can devastate your client’s tax situation, along with practical guidance for avoiding them.