I picked "free rein" because, well, that's the primary used one, but "free reign" actually works as well, and now i'm wondering how often I've maybe not noticed which one has been used.

Prostrate for prostate makes my medically trained brain cry sad tears of woe, mainly because it's something I deal with a lot at work (also, "spayded" and "neutereded" for "spayed" and "neutered" respectively, and the sometimes... aggravating infuriating truly baffling charmingly inept pronounciations of everything from drug names (both brand and generic. I thought the pharm companies designed the brand names so that they were easy for people to say?) to diseases to Things They've Googled And Are Now Obsessed With)