When you repeat a mistake, it is not a mistake: it is a decision.
- Paulo Coelho
A mistake is not being rational or being inconsistent with an articulated decision process. If there is a process and the decision-maker deviates from the process, it is a mistake regardless of your luck. If you don't have a well articulated process, you will not be able to say that you made a mistake. A more fluid process allows for ambiguity and thus makes it hard to admit a mistake even during bad performance periods.
Given there is a process, the real issue is whether the decision-making model is wrong. If you are rationally following the model, the focus is on whether the model of market behavior is incorrect. If you consistently follow a disciplined process but if the model's predictive power is flawed, there is a mistake. Unfortunately, most models of asset prices can only explain a small portion of the variation in prices. Hence, mistakes can be repeated and turn into bad decisions.