Sometimes (not often) I see mistakes in another correct solution but those can only be reported via text box.

You could add "Another "correct" translation is wrong" or "The English sentence is unnatural or has an error " to report problem when translating to English.

example: http://www.duolingo.com/comment/431395

• C'è qualcosa che non avrebbe voluto fare?
• My translation: Is there something that he wouldn't have wanted to do
• Another correct solution: Is there something that he would have wanted to do?