If there is a one letter difference (or if two letters are swapped), they accept it. If there is a two letter difference, they don't. If it spells a real word in either English or Spanish, even if it's only a 1 letter difference, they won't accept it if it isn't supposed to be there. I think this one is just an oversight by duolingo. Hope this is understandable.
I know, that's what I was saying. The "f" and "v" are right next to each other on the keyboard, so there is a lot of typo potential. However, Duo wanted to keep "knifes" from being accepted as a typo error to keep people from using it, as it is bad English.
I don't think anyone was claiming that it was proper English, only that it could have been accepted as a typo error.