No matter how miniscule the change you made to a mix that sounded good before, always go through the process of testing your new mix on a variety of devices / speakers.
I changed Flight by removing some low end EQ on the bassline and bumping up that track’s volume by 2 dB to compensate, and assumed that the mix would be fine. I even listened to it briefly on my monitors.
I heard the MP3 mixdown via my iPod through my car speakers for the first time today. The mix is horribly messed up, with the vocals being far, far too prominent.
Don’t take shortcuts.