I'm a stock trader and with the use of a mechanical trading system, I believe I've had some moderate success with respect to my return and drawdown preferences. Still, I remain mindful that my system is bound to fail eventually. With that, how have your properly backtested systems failed in actual trading? I believe that a lot of you seem to know how to properly backtest systems. So I'd like to know when these good-in-paper systems failed in actual trading. Was it because you arbitrarily skipped trades? Did you change some components of the system? Or perhaps the system just, well, degraded?