Overcooking food in a slow cooker is possible especially if you don't add enough water when the water evaporates from the slow cooker.
Make sure after the slow cooker has cooked the food to turn the slow cooker down from high to low or warm setting.
When the slow cooker is on high setting it will continue cooking the food at a higher temperature which can cause the food to overcook such as beans can get burnt and dried out.
If you turn the slow cooker down to low or warm the food will not overcook and you can keep it on all day or even a few days on warm to allow the food to stay warm and ready to eat.
Sometimes I'll make a big slow cooker full of beans and ground beef and then leave it on warm for a few days as we eat the beans and ground beef.
The food is already cooked and stays warm.