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]
(..) Do you like music? This is a real piece of shit. Oh. Well, tell me what you really think. Uh, just, uh, the linear and impressionistic mix... makes a very muddled composition. It's also a Winslow Homer rip-off, except you got whitey, uh, rowin' the boat there. Well, it's art, Monet. (..)1997 Good Will Hunting