When butter goes “off,” does it become unsafe or just lose quality?

Butter develops “off” aromas and flavors as a result of exposure to oxygen and moisture. “Off” butter may be discolored or moldy, and smell stale, cheesy, malty, or have a rancid “microwave popcorn” odor.
 
Although commercial processing removes unsafe bacteria from butter, it’s a good idea to keep it sealed and chilled in the refrigerator.
 
Source(s):
Assistant Professor Franco Milani, Department of Food Science, University of Wisconsin
Journal of Dairy Science: “The Effect of Refrigerated and Frozen Storage on Butter Flavor and Texture” (2008)
The Microbiological Safety and Quality of Food by Barbara M. Lund and others, 2000.