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]
(..) All right. School bus is clear. You got the green light. Watch this. Makes them shit. Go, go, go, go! Meth labs are nasty on a good day. You mix that shit wrong and you got mustard gas. Phosphine gas. - I think. - Yeah, exactly. One whiff will kill you. (..)2008 TvShow: Breaking Bad Title: Pilot Season: 1 Episode 1