This happens to me all the time. But what I find REALLY annoying is when the person I'm talking to doesn't know what it is I'm trying to say (when I can't get the word out straight away) and just stands there waiting...I mean...come on...are you not listening...are you stupid...you know what I mean...right...? No? Idiot!
Oh, so you don't find it annoying if they complete your sentence for you? Or do you find it helpful? I always complete the other person's sentence and they generally seem to find it useful.