My guess for the most common planning mistakes:
1) Not having an actual plan, only a goal. Essentially, just saying "I want to be X", and then waiting for it to somehow magically happen. As opposed to researching how people actually get from "here" to "there", what kind of tasks they do, which skills they need, and actually practicing those skills. In other words, not making the first step, but instead waiting for the "right moment", which somehow never arrives; or if it does, it will find you unprepared.
2) Expecting the whole thing to happen in one big step, as opposed to setting up your activities and habits so that they keep drawing you in the desired direction.
For example, if you want to get fit, a typical failure is to buy an annual ticket to a gym... and then never actually go there. (Unlike the previous example, you have actually made the first step. But then you wait for the second step to happen magically.) A more successful plan would be to simply start doing push-ups every morning; and perhaps think how to reward oneself for doing so.
Or, if your goal is to become a writer, a typical failure is to start writing your big novel... only to end up a few years later with hundreds of pages of horribly written text, which obviously doesn't have a future, but the sunk costs are breaking your heart. (Now the problem is that you have skipped a few necessary steps.) A more successful plan would involve reading other people's texts and writing exercises, at specified time every week. (Similarly for computer programming.)