Dried beans that refuse to get soft. This batch of black beans was in the crockpot for 10 hours, then simmering on the stove for another 1.5 hours and they're still crunchy in the middle.
This happens to me all the time. You either have hard water or old beans. Adding baking soda to the cooking water helps or check the "best by" date on the beans.
Or you could do what I've finally done and go
and buy low sodium canned beans, give them a light rinse, and use them.