Rancid fats are teh suck. They taste bad, usually even in small quantities. They are also highly toxic and carcinogenic.
So avoid them. Refrigerate your butter, store fats and fat-containing foods (nuts and the like) in cool places, watch those expiration dates, throw them out if you have to.
Irony - stomach cancer used to be a WAY larger problem in the past, before hydrogenated and trans fats started being used. But now we know that hydrogenated and trans fats are huge problems for cardiovascular health. BHT, BHA, hydrogenation = lower cancer risk, higher other risks. There is no such thing as a free lunch.