The makers "I Can't Believe It's Not Butter" were succesfully sued in 2011 by a consumer group who stated that not only could they quite easily believe it wasn't butter, but it did in fact taste like "shite margarine".
The makers have been allowed to continue using the name, as "Shite Margarine" was ruled as potentially offensive to consumers. However, as part of the ruling, if you look at the bottom of the pack, in tiny writing, each packet says:
"Notice - You may actually very well believe it's not butter".