The most common mistake is putting all money in one basket. The portfolio is really concentrated in just few options. This needs to diversified enough so that shocks can be absorbed. The second mistake is everybody wants to be rich very quick but such products are gimmicks and nothing more.
Every body must hold real physical assets, i.e. real estate, precious metals and other things.
The Other side of mistake is borrowing, borrowing more than the repayment capacity or spending more than present income stream expecting future wind falls. In financial market there is only today, and one must learn to live in the means what he has today.
Living safe and within the available means is best. When one has more he can always thinks of more but please don't spend today based on future expected income.