A few years ago, my accountant told me I was getting an $8,000 refund. Something didn't feel right, so I asked him to take another look. At first he reassured me everything was correct. The next day he called back. He'd missed a prior-year item that should have carried forward to the return. Actual refund: $28,000. I almost left $20,000 on the table. Not because I wasn't capable of catching it. Because I didn't know what to look for or what questions to ask. That's exactly why this community exists. Most of us hand our taxes to a professional and assume everything is handled. And to be fair, it usually is at a basic level. Filing correctly avoids penalties. But filing optimally means capturing every legal strategy available. Those are two very different standards. Most accountants are only required to meet the first. What's the most expensive mistake you've ever caught from a professional?