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]
(..) About Polyakov. There is a mole. If it's bad, don't come back. I want to remember you all as you were. Percy? Percy, did you mix this? Aye, I did so. You Calvinistic, penny-pinching Scot. Can you nae learn to take a bloody order? Nobody tampers with the recipe! (..)2011 Tinker Tailor Soldier Spy