It has happend to even the BEST parents. Suddenly you see your baby drinking from an old sippy cup of milk that you forgot about from a few days before. This recently happend to me. I peaked into the playroom and saw my son, Michael (age 2), drinking out of a sippy cup that was at least 24 hours old and possibly even 48. He took one drink, swallowed, said "Yuk", and threw the cup across the room. I opened the cup (hoping to find water inside) and saw old, sour smelling milk. It still looked like milk. It had not separated and curdled like it does when it is soooo old. (Like the cup of milk I found on the floor of the car recently.) So, what happened to Michael? Nothing. Michael was fine. He never threw up. He never had diarrhea. Nothing bad happened. Recently a friend's 20 month old daughter drank a full 5 ounce sippy cup of milk from the night before (about 12 hours old). This little girl was perfectly fine, too. Nothing bad happened. So, if your baby has had a little bit of old milk, don't panic, baby will probably be fine. Of course, you can call your doctor to double check and certainly call the doctor if baby starts to get sick. Just keep calm because this has happend to many, many babies.
Please remember that I am not a medical doctor and this is not intended to be medical advice. Call the doctor if you are worried about the baby.
----Annie Pryor, Ph.D.