"The Incredible Hulk Returns" is a TV movie that I'm very fond of, too. I must've watched it over 50 times when I was 14.

I've never really noticed the moment you are referring to, however. I think it was probably not a blooper-----Lou Ferrigno probably meant for the Hulk to have that look. The Hulk is child-like, let's not forget. He can easily forget his rage when he sees or hears something that he finds interesting or fascinating.