I enjoy playing Grammar Nazi with lots of folks when they use the "un" prefix, as it usually (but not always) means to reverse an action already taken:
Waitress: "Would you like your tea sweetened or unsweetened?"
Grammar Nazi: "It seems amazingly complicated to sweeten the tea and then unsweeten it again. How about you just bring me non-sweetened instead?"
Hilarity (or at least odd glances) ensue.