Not unit testing. It’s not that noticeable when you have a small project, but as things get bigger and more complicated, the work required for end-to-end testing increases exponentially.
Not setting a clear end goal before you start. I’ve completed many projects, but the one I didn’t had a fuzzy end goal that I told myself I would eventually define. I never did. If the goal is crystal clear, it will motivate you to keep going.