The Pokémon anime is an arguable case. Some people have very fond memories of the first 13 or so episodes and dislike anything that came afterwards, but others have been watching for over 10 years with no guilt whatsoever.
High School Musical. The title (and the Ho Yay-filled second movie) speak for themselves, really. Once the hype dies down, you'll deny having ever watched any of them...but they'll know. Oh, how they'll know!