This is a phrase often uttered by sound engineers during a recording session, especially when someone has screwed up or something just doesn't sound right. They are saying "trust me
we can mix things so that you don't hear it." The problem is that often times the fix ends up being just as bad as the problem was to start with. While its true that things can be fixed in the mix, when someone utters this phrase keep in mind that the better the ingredients that go into the mix, the better the end result will be. If you are pressed for time, or just can't seem to get something right, maybe you're better off to let them fix it in the mix. But if you are going along with this because you are being lazy remember that garbage in results in garbage out
Here's a list of things that people often try to fix in the mix and reasons why it can be a bad idea:
Wrong Pitch - while you might be able to bend the basic pitch to where you want it, you're probably going to mess up the harmonics.
Tempo Issues - it's very rare to find maintain a perfectly steady tempo through a piece, if you fix the temp for one scetion you are going to mess it up in another
Dynamics Issues - While you might change the volume of something, you aren't going to change the intensity of it.
All this being said, I'll admit that many a time I've used these very words. There are times when someone just isn't managing to get something, and you're better off moving on. There are times when you would better off investing the time in laying down more tracks rather than trying to fix that one little imperfection There are times when if you redid something it would have more mistakes in it than what you have just done.
When someone says "we'll fix it in the mix" just keep in mind that there is trade off. Try to be as informed as you can about what you are giving up and what you are going to get in return for fixing it in the mix.