I'm okay with people finishing words for me as long as it's completely obvious what I'm trying to say; it's when people just start repeatedly guessing that pisses me off. If you think you know what I'm trying to say but get it wrong, it's no big deal, just please don't try and guess again - A friend did this and made 5 or 6 guesses and still didn't get the message that maybe he should stop interrupting.