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]
(..) Well, not that big. My goodness. Marcello, come here. Hurry up. Why do they switch the "R"s and the "L"s here? Oh, for yuks. You know, just to mix it up. They have to amuse themselves. 'Cause we're not makin' 'em laugh. Let's never come here again, 'cause it would never be as much fun. (..)2003 Lost in Translation