Here on this board we can all find safe, reliable advice/instructions for a project we're working on but somewhere along the way we pick up terrible pointers that cause us more pain and work.
The main point that someone once gave me was that cotton stretches so if your points don't line up, just stretch it to fit. That might be true if it's just a bit too small but she demonstrated on something that had larger missed margins and the finished product was so out of shape, I wince when I look back on the project. Back then, though, I was certainly none the wiser.
What was the worst advice you ever got about quilting?