Once I was told:
Do not have a plan B. Because when you have one, you are more likely not to stick to plan A, and your attention is attracted to plan B to avoid plan A.
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On the other hand, if I would sometimes rely on plan A (which highly relies on other people's decisions – this is something you can't always influence because of their plans), I would probably crash and be broken.
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So... Is it good to have a plan B or not?