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]
(..) The Avatar Program is a bad joke. Bunch of limp-dick science majors. However, it does present an opportunity both timely and unique. A recon gyrene in an avatar body. That's a potent mix. Give me the goose bumps. Such a Marine could provide the intel I need, right on the ground. (..)2009 Avatar