The most feared words on any stock exchange are margin call. A margin call is made when a position is losing money and more money is required by the broker to keep the trade open. If and when a stock ticker moves quickly, there can be people whose borrowing levels literally bankrupt them as things get worse ... fast. Volatility can be either a blessing or a curse, but if you have too much leverage, it can break a trader.
Experienced investors such as Buffett eschew stock diversification in the confidence that they have performed all of the necessary research to identify and quantify their risk. They are also comfortable that they can identify any potential perils that will endanger their position, and will be able to liquidate their investments before taking a catastrophic loss. Andrew Carnegie is reputed to have said, “The safest investment strategy is to put all of your eggs in one basket and watch the basket.” That said, do not make the mistake of thinking you are either Buffett or Carnegie – especially in your first years of investing.