If the corn syrup mixture is boiled for too long, it causes the cereal base to get very hard when cooled. To prevent this, keep a close eye on your corn syrup mixture and don't walk away. As soon as it begins to boil, remove from the heat and continue with the recipe. Scotcheroos may also get hard if they're refrigerated. Allow cold bars to come to room temperature for about 30 minutes before slicing and eating.