I have had quite a few sewing "Ah HA!" moments in my life, but usually they come after the "Uh oh" moment - that time when your brain kicks in and says "Ohhhh noooo - you're going to have to rip that out/recut that/cry."
On of my biggest "Uh oh!" moments was when I learned to do a princess seam for the first time and didn't know that if you pre clipped the seam allowance, that it would be easier to match up the very curved bust seam. So instead I battled this beast for hours - pinning, basting, ripping - until finally I had what I wanted! It was at this point I realized I had sewn the front panel to the back side panel and not the front side panel. D'oh!
Lesson learned: always mark your pattern pieces with a little chalk or disappearing ink, clip your seam allowance on curved seams before you try to piece them together (after you make sure you have cut the right size, of course!) and take a freakin' break - when you're in the middle of a sewing disaster it's hard to get back on track without a brain break!
What's your "Uh oh!" moment and what did you learn from it?