Surprise ending? You mean they had the ending where the plant wins and assimilates the main characters, or something else?

I've heard that both movies originally had this ending, but Frank Oz decided that with the realism of film it was more depressing than funny, and so they went back and re-filmed the last 5 minutes of the movie. You can still find the original "plants take over the world" footage on youtube. It has some cool special effects.