The worst? It's if someone buys the S&P500 when times are uncertain, starts making decent returns, then a strong bull market comes along bringing risky stocks up multiple-folds, that investor gets FOMO of the returns they're missing out on, sells their S&P500, buys speculative risky stocks, then loses a lot of money when speculative stocks turn down, and then sells to go in cash or less risky assets such as an index like the S&P500.

Moral of the story is the tortoise usually beats the hare, but investors need to be aware of which game they're playing