I was day trading cable (GBPUSD pair) with a relatively large broker that on too many occasions failed to fill my limit orders claiming that there was no volume and constantly stopped me out even though I proved the volume was on the book.

I knew my strategy was sound, but just kept getting stopped out all the time, so instead of adjusting my risk ratio, I messed up big time and put in market orders without a stop loss. I had some remarkable gains, but the inevitable happened and my margin account was liquidated when the market went south, twice! I had got greedy and stupid and lost everything I had gained.

So I learned my lesson there!