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]
(..) l would have gone overboard, but Mr Dawson here saved me... ..and almost went over himself. - She wanted to see the propellers ! - Women and machinery do not mix. Was that the way of it ? Yeah. Yeah, that was pretty much it. The boy's a hero, then. Good for you, son. Well done. (..)1997 Titanic