I remembered hearing, long ago, that peanut butter doesn't tend to go bad but that when it does, it becomes very carcinogenic. Not knowing the details of this little fact, I turned to the internet and found a bunch of sources to explain what was going on.
The molds Aspergillus flavus and Aspergillus parasiticus grow on a number of things and grow especially well on peanuts. These molds release a chemical known as aflatoxin, which is a strong carcinogen and has been linked to liver cancer.
The FDA monitors aflatoxin levels in commercial products and it turns out to be the case that the big brand peanut butters (Skippy, Jif, etc.) have the lowest levels of aflatoxin. Also, if you go to a health food store and get freshly ground peanut butter, you're dumping tons of aflatoxin into yourself.
So buy big brand peanut butter and don't let it go bad.