One of my first real DIY projects during a summer in college nearly 20 years ago was replacing the rotted out basement bulkhead doors on the ~120 year old house I grew up in. I took measurements of the old ones, bought some nice tongue-and-groove cedar and high-quality hardware, and built the new doors in the garage. When they were fully assembled, I carried them over to install on the old stone frame. I took off the old ones, put mine in their place...and they didn't fit properly at all.

Momentarily baffled, I realized that, despite appearances, the old frame was actually not square, in fact it was a parallelogram. I'd measured the height and width and assumed it was square. The previous (experienced) carpenter who'd built the doors I was replacing had clearly noticed this, and simply allowed for the misalignment in his design. He built perfectly square-appearing doors that mounted to the not-square frame. I had to go back and rework mine considerably for them to fit without looking ridiculous. They're still there and holding up well, but I also still think of this lesson on a regular basis in my day to day life now.