Lack of stock market knowledge. No basic knowledge.
One of the most common mistakes is to simply start trading without any prior knowledge. Due to a lack of basic knowledge investors create the risk of losing a lot of money on the stock market.

Short-term Trading
Investors believe in short term trading and not looking for long term investment. Investors buy stocks and sell to early in the event of a rising and falling share price. They are fearful of falling shares and sell shortly. Causes they lose their  money

Investors put all his money in a single stock. Not Diversifying Risks
Investors invest all the money in single stock causes when the stock price falling investors getting fearful and sell stocks in heavy loss. Investors not making portfolio in Diversifying term. Investors need to create a Diversifying portfolio so that if one stock falling then other stock will cover the loss.

Buying Shares on Credit.
Small investors borrow money for a share purchase from a bank, friends, or family or a broker. The risk of large losses (for bank loans) and destroyed trust (for family loans) is far too high.

Buying high, selling low.
Investors buy stocks at high prices and sell in low when the stock price is falling. They do because of a lack of knowledge and not control on emotions. They take to much advice and invest money at a high price.

Investors doing over trading buying too many stocks (Frequent trading).
Investors buy and sell stocks frequently they are greedy and trying to make money very fast. They should know that no one can make money fast. Investors need to invest in the long term with fundamentally strong and debt-free companies.  

Buying (and selling) based on emotion.    
Investors make the biggest mistake not to control his emotion while stock price rising and when the stock price is falling. They buy and sell stocks in emotions. They should know that you can't make making from the stock market from emotion.

Following Recommendations Blindly.
Investors another common mistake is to take more advice and Recommendations Blindly and buy stocks on recommendations. Investors should take more Recommendations Blindly and make their own analysis.

Getting greedy.
Most of the time Investors are greedy he wants to make money in the short term. Investors ignore the real factor behind the stock market ups and downs and they follow the trend only, but investors should be careful and not be greedy in the rising stock market. Investors need to invest his time in analyzing the company fundamentals and management.