One of the biggest — and often costliest — retirement mistakes investors are making today

Target-date funds are all the rage among retirement investors, making it easy to spread money among stocks and bonds and to rebalance as conditions change. Total assets in this sector topped $1.1 trillion at the end of 2017, up from $158 billion at the end of 2008, according to Morningstar.

But is this too much of a good thing?

Vanguard Group, the fund giant, reported recently that 51 percent of investors in 401(k) plans with the firm put all their money into a single target-date fund in 2017, and Fidelity Investments has similar figures.

Read More: CNBC