You know, finding the cool thing for the finished project, before even measuring to see if it will actually work. So far I've made everything eventually work, and only lost thousands of hours of re-design time. LOL.
However, I've found that by doing it this way, I've saved myself thousands of dollars. First, we get an idea of what we'd like to do. Then we just put it "out into the universe" (some people might call them prayers and positive thoughts) and voila! The perfect thing just falls into our lap. Sometimes it takes a few days. Sometimes it takes a couple years.
I'm terribly patient when it comes to saving money.
It's also a good way to put off a project.