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]
(..) ANNA AND FONTANA/ Stupid cupid Hey, hey, set me free Stupid cupid, stop picking on me You mix ed me up but good Right from the very start Hey, go play Robin Hood With somebody else's heart You got me jumping like a crazy clown... (..)2001 The Princess Diaries