Measure twice, cut once!
This is a mantra drilled into every design student's head while in school. The moral being, better to be careful and correct at the beginning rather than fuck it up later on.
I made that fatal mistake in haste to begin a project I had toyed with in my head and sketchbooks for the past year and realized my error. But rather than mope about (which I did for a day or two), trying to "fix" my problem led to multiple solutions and opportunities. Now I just need to "measure twice, cut once" again.