The most aggravating part of mastering a song can be summed up thusly:
"Which sounds better? Mix A or (tweak tweak) Mix B? Mix A? Or Mix B?"
I find a final, mastered mix that I like, and almost settle on it, but in fixing one final thing I notice that something, technically, is incorrect. So I start fixing that, and in doing so the mix deteriorates into something that, although technically correct with no overloading or clipping, sounds crappy.
Tonight was one of those nights. All I needed to do was pull down the vocals, raise the bassline and call it done. But I noticed my final EQ plugin was getting overloaded on its input, and although I can't hear any clipping or buzzing, it's the knowledge that somewhere there's something incorrect. My solution was to add a limiter before the EQ, and that made it all worse. Booooo. Read more about Best Mixed Plans