Just, simply failing to follow the plan
This is usually my problem. Even if I did make a plan, I ignore it.
Something going wrong with the plan (unforeseen issues)
I make a plan and then some part of it doesn’t match reality, so I throw it out the window. Even if the project succeeds, the plan failed.
Tutorials
Tutorials are often terrible plans for real projects.
First we make a design that we know will work because we’re experienced enough to create a tutorial!
Then we implement that design without any problems because we knew what we were doing when we planned it!
I’m going to make a design of what I want!
Oh, turns out that what I want is (at best) difficult to implement. (And at worst it is incompatible due to some technicality.)