1. Start investing without any goal or plan.
2. Investing without proper research or study
3. Buying stocks at the time of selling and selling at the time of buying.
4. Short term trading by greed instead of long term investing by patience.
5. Following the actions of big famous investors, FII's (Foreign Institutional Investors) or fund houses
6. Following the public blindly
7. Investing in penny stocks
8. Taking advice from Uncle Sam for investing or taking advises from a wrong, non-qualified adviser.
9. Believing tips, analyst reports blindly
10. Investing on market rumors
12. Not having good understanding on the company, business, before investing in a stock.
13. Ignorance of necessary valuation methods to analyze a stock or analyze performance of a company.
14. Investing without discipline and patience
15. Ignorance of the basics. I.e. meaning of investing, what is a stock, how stock market working etc.
16. Ignorance of factors that capable to lead a market to bear phase or bull phase
17. Buying through IPO's only
18. Unable to evaluate a stock to identify to buy or sell
19. Investing only on hot sectors or companies from a fast booming sector like internet, real estate etc.
20. Marrying stocks with emotional attachment
21. Borrowing money for investing in stocks
22. Using credit cards for investing
23. Being panic when fluctuations happening
24. Monitoring the portfolio multiple times in a day or week
25. Not knowing portfolio diversification
26. Over diversification by 'n' number of stocks
27. Selling good stocks to meet temporary money requirements.
28. Over enthusiasm and expectations
30. Investing only to avoid tax
Hope the above list will help an investor to take a self assessment as well as help to avoid such mistakes in the future. Be an intelligent investor.