That reminded me of another one! I usually leave butter (salted butter, which matters) out on the counter in a rubber-sealed butter dish. And usually I use it all up without issue. BUT, sometimes I don't use it quickly enough (within a few weeks), and then things taste funny even if the butter still looks OK. Things taste a little sour, or metallic almost, and get trashed. And I don't usually smell or taste the butter before I use it if it looks OK - though I've started giving it a sniff if I think it's getting old.