Every movie, no matter how acclaimed, will have some bad reviews. Just like how even the worst movies will find someone who liked it to whore out his or her quote on the DVD cover. There are agenda critics out there who will bash any movie that doesn't fall into line with their "moral" or "political" or "cultural" beliefs, or overpraise movies because they do fit the bill they're pushing.
Maybe the guy who gave a positive review to Dungeon Siege will find something wrong with The Dark Knight.
Hell, Rotten Tomatoes used the "review" of Mission: Impossible III from some nobody on the internet who bashed the movie for the sake of bashing Tom Cruise, even though if you read what he said it was obvious that he never even saw the movie.