Wow - That's a very broad question! Well for individual investors, I would say that inadequate or incorrect diversification would be one of the most common errors. I know of people who invested in a basket of Irish Banks back in 2006/2007 thinking that because a number of banks were included that they were diversified. This was not a good strategy because the whole banking system crashed in 2008. Diversification requires looking at your entire portfolio (and that includes your personal home). If for example you own a mortgage free home valued at 350,000 and you other assets are valued at let's say 200,000, then you should not invest further in property.
Another common error is to jump on the new fashion - e.g. everyone starts talking about Gold saying that it will go to $15,000. Thats what people were saying when Gold was on a bull run, but soon after the price fell about 30%.
Over-trading is another problem, unless you are making a living from day-trading. (see my guide at http://www.learnaboutwallstreet.com ). Over-trading adds considerable cost in the form of broker commissions etc. It is not a good strategy in the long term, and it also adds to your tax drag in most countries.
Biggest issue: Emotions
