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[Al Saqr]

Hindsight bias, sometimes called the I-knew-it-all-along effect, is the inclination to see past events as being predictable and reasonable to expect, perhaps because they are more available than possible outcomes which did not occur. Subjects also tend to remember their own future predictions as being more accurate than they were after the fact. People are, in effect, biased by the knowledge of what has actually happened when evaluating the likelihood of what has already occurred.

Hindsight bias has been demonstrated experimentally in a variety of settings, including politics, games and medicine. Prophecy that is recorded after the fact is an example of hindsight bias, given its own rubric, as vaticinium ex eventu.

It has been shown that examining possible alternatives may reduce the effects of this bias.

I think you can easily understand now why you shouldn't use a trading system only because you backtested it.