I had a friend in high school who stuttered, and I would sometimes do this for him. Whenever I said the word it would help him complete it. I also worked tech support for a cable company, and one time I had a person who stuttered call in. I fell right back into finishing words for him. I remember that call in particular because at the end of the call he sounded so grateful that I helped him. It made me realize he was probably super nervous to call in, and that a lot of people in call centers probably would treat him like shit because it makes their calls go long. I was thinking about it tonight and wondered if it was actually rude to do that or not. After reading these threads I guess the conclusion is sometimes yes and sometimes no.