How could something as harmless as rice cereal be potentially dangerous? 

Feeding a baby that is under 6 months old ANY SOLID food  COULD BE dangerous, and rice cereal is often the first solid food parents will offer their baby. First and foremost Most babies under 6 months has a digestive system that is not fully developed; therefore anything other than breast milk or formula will NOT be digested properly. So, while that little bite of mashed potatoes or rice cereal seems harmless enough it could POTENTIALLY be damaging to the baby's digestive system. All that a baby NEEDS, to properly nourish them is breast milk or formula until 6-12 months of age. The reason we are focusing on rice cereal or any baby cereal for that matter,  is because it lacks the nutrition, so adding it reduces the nutritional density of their diet. It is safe, however, to start introducing solids to babies beginning at 6 months, but it is generally unnecessary. Breast milk will change as the baby grows, offering them everything they NEED for complete nourishment. If the formula is not giving them the needed nutrition, as it never changes, then only the parents and a nutritionist can decide together what it is the baby needs to add to their diet in order to properly nourish them. [1]

Saying that YOU fed YOUR baby rice cereal under 6 months and they ended up fine isn't a valid argument. There is no need to justify or explain why you do what you do. I realize all babies can develop at a different rate, but the general rule is that breastmilk & formula is all that is needed for the first 6 months of life. If you feel your baby is showing signs that they are ready for solids look into Baby Led Weaning.

 "Health experts and breastfeeding experts agree that it’s best to wait until your baby is around six months old before offering solid foods. There has been a large amount of research on this in the recent past, and most health organizations have updated their recommendations to agree with current research. Unfortunately, many health care providers are not up to date in what they’re telling parents, and many, many books are not up to date." [1]

Rice cereal potentially could cause-

1. ~Obesity- Dr. Alan Greene, a pediatrician at Stanford University, actually equates feeding white rice cereal with feeding a baby sugar or soda, according to ABC News. Dr. Joseph Mercola, a natural health expert, agrees that giving babies white rice cereal could lead to diseases such as heart disease and diabetes, due to an increased risk of obesity when infants taste starch as a first food and therefore grow up craving foods made from white flour, such as breads and cakes. [4]
2. ~Constipation- Rice cereal is usually fortified with iron, but some experts argue that all that extra iron isn't needed in infants if they are otherwise well nourished and too much iron can cause constipation. According to KellyMom, babies who are breastfed for the first six months of life per the American Academy of Pediatrics guidelines may not need additional iron, unless the child's pediatrician determines that breast milk or formula alone isn't providing adequate iron.
3. ~Lack Nutrients- Baby cereal is filling for babies, but it doesn't offer a great deal of beneficial nutrients for the amount of calories it provides. Your physician may still recommend rice cereal, especially if your baby is deficient in iron, of which it is a rich source. According to Dr. Greene, however, many babies are better served with brown rice cereal, since it is also rich in protein, essential fats and minerals. According to KellyMom oatmeal would serve as a better first choice because it offers more of a variety in nutrients and protein.