In most crafts, the first lesson learned is “measure twice, cut once.” Well this applies to sewing too. And not just to cutting. Today I learned to”measure twice, draw once.” Otherwise you'll have lines that seemed to be parallel when they were drawn but somehow end up crossing as soon as you start to sew along them.
Also, on a related note, as a bonus lesson: Always, always measure and mark your line in three or more places if you want them to be even slightly straight and even.
So today's lesson is to measure, take a step back, then measure again. Otherwise you'll waste precious time ripping out crooked seams and washing out the lines it took you hours to make. Measure, measure, then measure again.