heh the first time i took on a project as a new homeowner (building a deck) i learned this lesson. I spec'd everything out on paper and covered all the bases and felt pretty confident. Then I purchased the wood and so many pieces weren't the size advertised and i don't mean how a 2x4 is not really 2 inches by 4 inches. Some pieces were 3/4" too short, others too long, some straight but most weren't. It took so many trips to/from home depot and went over budget and wayyy over time.

oh and post holes, some are easy and some you hit mystery chunks of concrete(!?). It was like archaeological expedition trying to get deep enough holes in the ground.