Regardless of how hard I try to plan ahead, home repair jobs always require multiple trips to the hardware store. Usually, the extra trips are due to me underestimating the size of the project I'm taking on, or due to me discovering that the problem is more complicated than I had initially thought.
Adopting a "two is one and one is none" mentality has helped to minimize the number of sheepish encounters I have with the hardware store cashier. However, because I usually redo my first attempted fix, I should revise this to "three is one". Despite routinely being concerned that I'll buy too much at the hardware store, I rarely regret having bought too much or wanting to return tools or supplies.
While I suspect this is inherent to DIY, I wonder if pros go through the same process?