I recently read an article that has some tips on a few very common mindset traps you should avoid. As I read through them I realized that I have had them all at various times, and it never ends well.
The first bad habit is to assume your tracks will sound better once they are mixed with the other tracks. Now I actually don't think it's 100 percent wrong all the time. But in general, you should make each individual track sound as good as possible and not count on the "bad" to be less noticeable when in a crowd with other sounds.
One exception that comes to mind though is how sometimes individual tracks, when soloed out of a mix, may not sound great on their own after you have applied EQ and other effects to individual instruments to allow them all to be heard in a mix. For example, it is not uncommon to scoop out a band of frequencies of an acoustic guitar to allow a piano to poke through the mix better. The parts of the guitar that sound good blended with the piano may be the high parts, and the piano may fill the middle frequencies to provide a balanced mix. But when soloed, the guitar might be missing a bunch of the middle frequencies that the piano is providing, making it sound too high and thin all by itself. In this case, the guitar really WOULD sound better in the mix. There are lots of examples like this all over the frequency spectrum. So keep that in mind when avoiding this particular bad habit.
The next one was to assume your mix will sound better when it's mastered. While it might be true, you should never count on it. In fact just never think this way at all, ever. Just mix your songs as if your mix is what will be heard by the public. Then all you have to do is leave enough headroom for a mastering engineer to do his or her job to make it even better.
The last bad habit is the assume everyone will listen to you mix in stereo. The idea here is that when all your tracks are panned, they may sound great. But when "folded" to the center things might very well not play well with each other - with frequencies possibly cancelling each other out or combining to become too loud, or both. So it is often recommended that you test your mixes in mono to see if you have these phase problems. On the other hand, I think in this day and age, it is actually true that most folks will be listening in stereo. Also, I remember reading an article a few years ago in Recording Magazine by a producer who said he didn't care (and his name was not "Honey Badger:)). He said if someone listens to his mix in mono and it sounds all whacked out, so much the better. It might be interesting to the person who dared not listen in stereo. Of course, I paraphrase.
Anyway, read Graham's article about these three habits here: http://therecordingrevolution.com/2012/08/27/3-dangerous-assumptions-in-the-studio/