Something happened to me last month that may never happen again. I was given the opportunity to calculate the return that one of our newest clients had achieved in her mutual fund over the past three decades.

What’s the big deal? you ask. Isn’t that my job?

Well, yes. What usually makes this so difficult is that in most cases investors make changes along the way. They change jobs, change financial planners, choose different funds, change their contribution amounts, take withdrawals, or have their funds killed. But even if I have all of these transactions in all of the various funds and the exact dates that changes were made, it would be next to impossible for me and my desktop calculator to determine the rate of return that the investor achieved. There are just too many moving parts.

So we normally rely on DALBAR for this data. They do a great job with complex equations like these. At the end of 2010 they determined that the average equity investor vastly underperformed the market again over the past 20 years achieving an abysmal 3.84% annual rate of return while the market topped 10%.

This client, however, had made exactly 330 monthly payments (27.5 years) of $100 per month as a direct withdrawal from her checking. This money was invested over that entire time in one mutual fund that had not closed or changed or merged. All of this is extremely rare. On the monthly statement it was confirmed that the client had invested $33,000 of her own money. I was able to take the current balance, which was just over $100,000 today, and compute that the average, compound annual rate of return was around 7.2%.

But when I computed this for the client she and I had different reactions. I was impressed—I was expecting something less than four percent. She was upset—she was promised market returns and has not gotten them.

Here’s the take home: The market gives a rate of return over long periods of time of around 10% to those who are in the market and capture that return. But the market return is not achieved by most investors due to either their behavior or the behavior of their mutual fund manager.

In this case, the mutual fund held around 200 single stocks and turned them over at a rate of around 80% per year. Since we know in this case that the investor’s behavior didn’t change during the entire 330 months, it must have been the behavior of the manager. How bad or how expensive does a manager have to be to get 7% when the market gets 10% during the same time period?

A better question: When are we as investors going to hold them accountable?