Investing isn't a game. If you screw up, real money is lost.

As a broker and supervisor for a major discount brokerage firm for close to two decades, I saw it firsthand. People lost their children's college tuitions and even their own retirements.

Now, I'm no longer an advisor, and the following are simply my opinions. With that in mind, here are the six most common mistakes investors make.

1) Having a Poor Plan – or No Plan at All

Having no plan is just foolish. But a poor plan may be even worse. Imagine someone saying, “I'll just buy XYZ stock. It's been doing great. That will fund my retirement.” That should give you a sense of what I'm talking about.

Whether or not they have an advisor, every investor should have a written plan that lays out their specific objectives, including details concerning time frame and risk tolerance.

Also, as an investor's financial situation changes, the plan needs to grow and evolve accordingly. Keep in mind that life expectancy continues to rise, so a portfolio needs to last longer than most people think.

Remember, too, that each person's circumstances are different. A cookie-cutter approach won't work, which is why I'm not a fan of robo-advisors.

2) Betting That a Bull Market Will Last Forever

Don't gamble on a bull market in stocks lasting forever.

That's the message you'll get if you watch CNBC, which is full of permabulls. If you must watch CNBC, I recommend doing so with the mute on.

In reality, bull markets aren't eternal. That's also the reason I'm not a fan of S&P 500 index funds. If a financial storm hits, you have absolutely no chance – you'll go down with the market.

3) Trusting an Advisor Implicitly

Some of you probably have a professional advisor, and that's great. Just remember the words of Ronald Reagan: “Trust, but verify.”