I don't have the issue anymore, but didn't Empire predict that Ice Age 4 would have the 2nd biggest box office of the summer, after the Dark Knight, but more than the Avengers? At the time it seemed odd, but now it just looks plain stupid. [8|]
I'm presuming you mean most successful of the Avengers line up. Considering Spiderman was released 2002 and made 820+million, a prediction of just over half a billion by Empire on a film that contains multiple Superheros (well established now, each with films under their belt) was not their finest moment.
Then again an estimation is just that: and estimate. No point in getting too riled up over guesswork.