It’s been a wild year for investors, with a stunning 30% drop in the S&P 500 (^GSPC) — and then a rise back to the all-time high we saw right before the Covid crisis took hold in investors’ thinking.

Since March, there’s been a flood of new investors getting into the market, joining investing apps Robinhood, TD Ameritrade, Schwab, and others — and retail investors have actually managed to capture many of the gains as the market surged back its March lows.

Datatrek’s Nicholas Colas, an industry expert and hedge fund veteran of SAC Capital, recently outlined 10 rookie investor mistakes in his daily newsletter — mistakes he’s made himself and seen done often by others during his 30+ years of experience.

“Three-star Michelin chefs still burn themselves, and master carpenters will occasionally hammer their own thumbs,” he wrote. “It happens.”

But understanding them can help repeat pitfalls. Here are a few from Colas’s list.

1. Plan before, not during a crisis

This is pretty much Planning 101 for anyone and anything, but Colas notes that many investors don’t plan their downturn moves until they’re in it. Mike Tyson’s “everybody has a plan until they get punched in the face” truism aside, having a winning approach — Colas’s is buy every 5% or more drop during a crisis — has worked well so far for the last two crises.

2. The stock’s price changed for a reason

Colas noticed that a lot of people attribute stock price changes to “general market action.” Colas says hedge fund manager Steve Cohen taught him that there is always a reason, and the real question is whether it's worthwhile to find out, because that takes precious time.

3. Policymakers play by their own rules

During the beginning of the coronavirus crisis, the Fed and the government did things they have never done before in terms of providing financial relief and stimulus for the markets, businesses and individuals. Colas notes that it’s a rookie mistake when people say “they can’t do that!” They sure can — and watch them.

4. Valuations don’t matter

"Math is not an investment edge," Colas said Steve Cohen used to say. Everyone knows stock valuations, which are public. The important part is "why" it's cheap or expensive and "how" that changes.

5. Misunderstanding time frames

Two of Colas’s mistakes can be combined because they both involve misguided thinking about time frames. A lot of people say things like “this won’t end well,” and Colas points out that bad endings are usually just a potential starting point. (Obviously, in cases like Enron, that’s not true necessarily.)