I don't think it's possible to overfeed a BF baby because
they have to work for their food so they're not going to bother to do it any more once they're full! It's possible to overfeed a FF baby because milk usually drips out of an upturned bottle, albeit slowly on the slower flow teats, so you can convince them to take just a little bit more even without realising you're doing it. Either way though, if it's not a normal thing baby will probably spit up the extra. If it's a regular thing, baby will get used to feeling over-full and come to believe they're not full unless they're stuffed to the gills... kind of like me and chocolate.
A baby could overeat if there was a medical problem like reflux possibly, as the milk might soothe the pain so they do have an incentive to eat too much. In a normal healthy baby though, not going to happen! Anybody who's struggled to feed a jaundiced/very sleepy newborn can attest to the fact that it's very difficult to force feed a BF baby