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]
(..) Soap. - Sorry? - I make and I sell soap. The yardstick of civilization. Tyler Durden. Did you know if you mix gasoline and frozen orange juice, you can make napalm? - No, I did not. Is that true? - That's right. One can make all kinds of explosives with simple household items. (..)1999 Fight Club