More than a few in the audience left the theater before Vader's breathing could be heard at the end of the closing credits. Later on there were people who'd say things like: "What? You could hear Darth Vader breathing in the end credits? Where? You sure about that?"
This is why I almost always stay until the end credits are finished rolling. Long before the Marvel movies existed, not remaining in your seat for the entire credits sequence could lead to missing out on some interesting things.