I lost a close to thousand of dollars in financial/stock market. The big mistake I did was invested in futures and option. The price of the stock I bought kept going down and me silly was averaging it out and trust me at the expiry date it was close to penny. The biggest mistake was I didn't know much about futures and option and just randomly went ahead with buying a stock which was down 20% thinking that within one months time it will go up but my bad luck and I lost  close to 1k dollars. This is the biggest lesson I learnt that before investing anywhere first do your homework and than start. So, for me I would never try investing in future and options that is tied to stock market.