I don't have any of the "use master ball on weak pokemon" or "save progress lost" stuff. I'm usually good at saving and I save often, especially before gym leaders/legendary fights/catching something rare (like a shiny). But I remember once in Crystal, I ran into a shiny Ratatta. I knew about the red Gyarados and thought that was the only Pokemon like it, so when I ran into the shiny ratatta, I though nothing of it and I either ran from it or defeated it. Since then, I never ran into another shiny Pokemon, at least until 4 days ago.
You could also say that I used my Master Ball on Giratina in Platinum. I really didn't want to use it since I wanted to save it for something worth while like Cresslia, Mespirit, a roaming legendary bird or Arceus when it comes out. But I got fed up with it escaping all my Dusk/Ultra/Timer balls and used it. And now I'm struggling with catching that dang Zapdos and Moltres.