Mistake(?) at "Il y a un fil qui ne sert à rien."
The right answer was supposedly:
There is a thread that is not useless.
That's one negation to many I guess. Shouldn't it be:
There is a thread that serves for nothing.
There is a thread that serves this question at http://forum.wordreference.com/showthread.php?t=558464. Well, in relation to 'ne sert à rien'. The example sentence there refers to an action (crying), not an object. And there is a song by Elisa Tovati - Ça ne sert à rien d'aimer - again referring to an abstract thing, love. BTW, I would go for 'thread that serves no purpose' over 'thread that serves for nothing'.
You're right. That's an error on duolinguo's part and you should report it. In addition to that: I'm not 100% sure whether the expression 'ne servir à rien' is commonly used with concrete objects. Any native speaker around who can shed some light on this?