One of the benefits of the internet is that we can blast and complain about anything without any fear of anyone calling us out on it; we are anonymous. Sometimes we are right to complain. Other times we are dead wrong. One of the best examples of fanboys being ass-backwards wrong was the outcry after Heath Ledger was cast as the Joker. This compilation photo sums up some of the backlash.
The only other example I can think of us fanboys being so universally wrong is Daniel Craig as 007. I personally always liked Craig and always thought he would be great but at the time, I was still cheering for Clive Owen.