So sayeth the man who really really wishes he'd measured three times....
No real harm done, save several hours of wasted time and a slightly wounded pride.
Skimping on the details (because there are a lot of them, and you probably don't care, and it's late and I'm going to get up early and try again), I attempted a little carpentry project today. My dad was a carpenter — and a firm believer that praise ruined a child. So carpentry's kind of a loaded subject for me, and if I haven't done any for a while, I tend to think I suck at it, like my dad always said I did — and to forget that I'm actually pretty good at it when the old man isn't watching over my shoulder and telling me I'm shit.
This particular project has an obvious and easy way to do it — if it were being done by someone skillful. The way my dad would have done it. I came up with a more convoluted way, involving certain apparatus designed to help people without such skills. It turned out (because I measured it wrong (or rather, measured the wrong thing about it), twice) that the apparatus itself got in the way of the thing I was building actually performing its function. So most of the way through the job, I ended up having to tear out all the work I'd done today.
On the other hand, in the process of building it I discovered that Oh, yeah — I do have the skills to do it the way dad would have. So early tomorrow morning I'll go back and do it the way i should have in the first place.