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]
(..) I told my brother to watch on the stove. All day long the poor guy has been watching helicopters and tomato sauce. I had to drive over to Sandy's, mix the stuff, then get back to the gravy. Do you think you can come over, fuck me, then leave? - Got someplace better to go? - You all right? (..)1990 Goodfellas
(..) - Yeah? Yeah. Of course. It took me a week of sneaking around to unload the Pittsburgh stuff. But when I did, it was a real score. I used Sandy's place to mix the stuff. Even with Sandy snorting more than she mixed, I saw this was a good business. I made $ in my second week. (..)1990 Goodfellas