From Wikipedia (good to know)

``Raw kidney beans, and some other beans, contain the toxin phytohaemagglutinin, which is destroyed by boiling for at least ten minutes, but not by the lower temperatures of a slow cooker, so dry beans must be boiled prior to slow cooking to avoid poisoning. Even a few beans can be toxic, and beans can be as much as five times more toxic if cooked at 175°F (80°C) than if eaten raw[citation needed], so adequate pre-boiling is vital. Cases of poisoning by slow-cooked beans have been published in the UK; poisoning has occurred in the USA but has not been formally reported.[8]

Slow cookers do not provide sufficient heat to compensate for loss of moisture and heat due to frequent removal of the lid, e.g., to add and remove food in perpetual stews, (pot au feu, olla podrida). Added ingredients must be given time to cook before the food can be eaten. If the food is allowed to cool below about 158°F (70°C) and not reheated, toxic bacterial growth is possible; some toxins, once present, are not destroyed by later heating.``