With volatility returning to the markets, investors should take a look at the biggest mistake made by famous activist investor Bill Ackman and avoid repeating it. That's what Daniel Crosby, a specialist in behavioral finance, recommended in an interview with Real Money.

Ackman's Pershing Square hedge fund unveiled a $1 billion short position in Herbalife (HLF) - Get Report back in December 2012. And this past July, he said that Herbalife's business model is finally on a track to "disappear" as its distributors begin to flee what is quickly "becoming the least attractive multilevel marketing company to work for."

The equally famous Carl Icahn has taken the opposite view and is long Herbalife's shares, a position that Ackman has said helped the stock climb more than 100% since its December 2012 trough. The two investors have exchanged heated words about Herbalife's true worth since then.

Crosby, who's president at Nocturne Capital and co-author of best-selling book Personal Benchmark: Integrating Behavioral Finance and Investment Management, said that Ackman is "a great investor who has let emotion get in the way. He has picked a personal vendetta -- he's getting crushed and losing billions of dollars for sticking to his point. It's become a personal fight with someone he dislikes, and not a rules-based decision. When it does become emotional and it does become personal, that's when you get in trouble."

The expert said that Ackman's biggest mistake is that he has become a "stranger to his own rules. He needs a greater reliance on rules and greater reliance on objective folks around him -- less contentions and less ego. He has made it very personal, and it's impossible to personalize something like this and be objective about it."

Still, Ackman has a great track record as an investor, which has helped him amass his wealth. This means he can afford to be wrong more often than the average investor. But with volatility sure to return after a long period in which the markets have been unusually quiet, Crosby said that it's important for the typical investor to "remain calm and stick to the rules."

That's easier said than done, but there are ways to achieve it. The first thing to remember is that emotion itself is the signal that something may be wrong. Crosby said that his favorite studies show that stress induces a 13% drop in IQ. This supports the idea that people should stick to rules to ensure they avoid big mistakes.

"The emotion itself is the cue," Crosby said. "Investing should be boring. Emotion is your cue to walk away and do something different. This is why we advocate that people work with a financial adviser." He said a financial adviser has the clear head and the objectivity needed to prevent the investor from acting out of emotion.

Of course, with a raft of central-bank decisions on the way this month and high political and geopolitical risks, market volatility is returning. Crosby said that to cope with it, "one of the most powerful things investors can do is understand a little history."

Since 1928, the U.S. economy has been in recession almost 20% of the time, which means the average American investor experiences between 10 and 15 recessions in his or her lifetime. "So, volatility is the norm -- but if you look at that period, we had amazing returns notwithstanding the fact that there was a lot of variability between those returns," Crosby said.

Corrections of 10% are absolutely normal, and simple rules such as diversifying across asset classes, sticking to the rules and staying the course are the best ways investors can protect against volatility. And, of course, avoid repeating Ackman's errors.

"The first clue that you're about to make a mistake is that there's a strong emotional undercurrent of fear or greed to your decision," Crosby said. "The best financial decisions are made in a cool, rational state. If you feel yourself overtaken by emotion, it's time to take a step back."

Editor's Note: This article was originally published on Real Money at 9 a.m. on Sept. 13.