Had a major ugly on May 6th 2010 when I placed a long on EURJPY and had an SL at 119.00, went to bed after a long flight and woke up to an empty acc. Turns out I had placed my SL at 11.9 not 119.00 and it was the night of a 900 pip drop thanks to the Dow 1,000 pt crash due to robots going nuts I believe.
Rule number 1; ALWAYs check you have your SL placed correctly.
The number one problem is without doubt taking too much risk. But as the saying goes, take care of the downside and the upside will take care of itself. In other words, think about what you're prepared to lose, not what you want to make.
My worst day trading would be about 4 years ago. I was long GBP/JPY (which was the wrong thing to be doing) and took a £13k hit in about an hour. Think I managed to end that day down about £7k.....
It was not as bad however as when I travelled down to London to buy a car (Ford Focus ST3) paid £12k cash only to find out 3 days later that it had fake VIN numbers, reg plates, log book etc... That experience was worse, trust me. My car trading carrier did not last long.