Expired mayonnaise will typically taste sour once it has gone bad. That’s because mayo tends to become acidic after it's gone rancid so “sour” is the best way to describe the taste. If, however, your mayo has expired but still tastes, smells, and looks fresh, it is probably safe to consume provided that you do so within the recommended time frame of two to three months past expiration for store-bought mayo and two weeks stored in the refrigerator for homemade mayo.