GMs definitely need to enforce strict "in character" play, eliminate "across the table" chatter, and by doing so - set the scene for an awesome role-play experience!
Purple deer have begun to appear near the ruins of a fallen city, or other place of power. Hunting the deer is absurdly easy, as their coloration makes them easy to see. The heros prepare a feast from a freshly killed deer find out several hours later that the meat of the new purple deer is poisonous. This can range from cramps and vomiting to the truly sinister and lethal.