Perhaps the biggest mistake I see is blindly buying index funds. Indeed for the past 15 years, broad market returns have been steady and lucrative. But I wonder how the next 15 years will fare. Current market valuations are at historic highs, which sets the stage for losses when volatility picks up.

Also, index funds are usually seen as very diversified, but over the past few years, they've become top heavy. Just 10 companies represent roughly 40% of the S&P 500. I suspect that quite a lot of investors are unaware of this index concentration risk.

It's a double whammy. Large shares of portfolios are concentrated into fewer stocks, which trade at very risky valuations. Yet so many retail investors and professional advisors alike continue to choose index funds for equity exposure. I find this situation so bizarre. It's like no one even cares about the details of the companies in which they invest