If you forget to brush and flush, no problem—likely nothing will happen to your toilet. However, I wouldn’t recommend leaving a cleaner sheet in the bowl for extended periods. First, it’s not designed for that purpose. Even though eco-friendly cleaners are made with mild ingredients and don’t contain bleach, remember that your toilet may have rubber or plastic components that could deteriorate under prolonged exposure to chemicals. If you have kids or pets at home, ensure they don’t have access to the toilet when a cleaner sheet is dissolved in the water.