I was making a new sled for the table saw that addressed some deficiencies I'd found in the old one. One thing I wanted to to was make it longer front to back so I could put the fence in from the part that I pushed with that I wouldn't be leaning over the saw at the end of the cut, and to help keep my hands away from the blade.
So I carefully measured and cut a piece of Baltic Birch for the base, marked it up, and was all set to start drilling and routing various slots and holes when the awful realization dawned on me--while I had precisely measure it, I had precisely measured it THE SAME SIZE as the one I was replacing.
Lesson learned: Think first, before you measure.
Didn't cost me anything but a little bit of plywood that I'm sure I'll find a use for eventually, so it's a cheap lesson, but I figured it was worth sharing.