Playing golf with clients is one way to 'mix business with pleasure'. [=to do something enjoyable that is related to your work]
I accidentally 'mixed up' the two files. [=I put each of the files in the place where the other one should have gone]
He 'got mixed up in' a plan to destroy the government. [=he became involved in a plan to destroy the government]
(..) We'll only do unto you for a moment, and it's no more than we would do unto ourselves. Girls, I want these books rearranged. They look too much alike. They're too pat. Mix them up. No, you mustn't mix them up. The books on this shelf pertain to empiricism, and on this shelf, materialism, (..)1957 Funny Face