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]
(..) Get those bellows working. Go! Bilbo! Up there. On my mark... ...pull that lever. Balin! Can you still mix a flash-flame? Aye. It'll only take a jiffy. Come on! We don't have a jiffy. Where's the sulfur? You sure you know what... ...you're doing? (..)2013 The Hobbit: The Desolation of Smaug