Not too long ago, I rambled on about the necessity of accurate measurements (and arithmetics) when building anything. Well, turns out that some of my measurements were correct and that my screwups were the wrong ones. see, I had taken measurements of one unit, doubled it and cut along that number. Then, when I installed the central support, I went along with the original measurement. When I looked at the finished items, I realised that the central piece was not centered after all and thus, moved it.
Turns out that, not only was it not centered, but that my original problem had nothing to do with measurements, but basic math: my mistake lay in the doubling of the lenght... I was wrong by about a couple of inches. Too long. My original placement for the central support was correct after all. I only noticed when I placed it on the floor and put the bookcases on it.
Back to the drawing board. Well, the skillsaw actually, since I have to cut them down to size!
This is where both humility and a strong ego comes in. And a solid sense of humor.