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]
(..) Then we're ready. Everything is in place. After I do the switch, how long before it goes off? A couple of seconds to let the chemicals to mix. Now, remember, when you enter the building, you'll see a door with a card reader. You will take the sergeant's keycard and buzz open the door. (..)2011 TvShow: Dexter Title: Talk to the Hand Season: 6 Episode 11