I don’t mean if it’s an emergency, or if you want to clarify something before someone gives more information (eg you’re talking about Jim and they want to check which Jim it relates to). I also don’t mean if it’s an accident and you don’t realise they haven’t finished speaking, and I don’t mean a situation where you’re very short on time. Also assume the person doesn’t have additional needs. This just relates to having a normal conversation in a relaxed situation.
YANBU: given the above caveats, it is rude to start talking over someone who was clearly part way through a sentence
YABU: it is always fine to talk over someone if you want to say something