The reason I am asking is because I want to know how difficult it is to create a system that backtests well but performs poorly in actuality. I created a system that profits almost $1 billion US dollars if I had traded with it for the past 10 years starting out with only $50,000 US dollars. This is an EOD system that buys at the next day's open price.
That's very easy to do. Some of the things to look out for are
1 - ensure that your code is not looking into the future (use code check in editor)
2 - be sure to backtest with both in-system and out-of-system data
3 - be sure that your backtest uses data for the type of market you want to trade (i.e., bullish, bearish, flat, a combination of all, etc.).
Be sure to read the portions of the Users Guide regarding backtesting and there are some good articles and books (e.g., Pardo's book) available as well.