It's so unfair.

I trapped white queen, took her, but I moved my knight on wrong square. The square was legal and save.

http://en.lichess.org/training/8592

which square? I think a4 is the best because it allows you to come back to b6 and attack the pawn in d5.

I went Nb7. When I hit the queen, I relaxed. But the main task (hit the queen) was done. As for me it's so unfair. XD

Na4 and Na6 are both accepted by the way :)

Wow! I didn't see this. Funny! Seems like I chose the worst move.

But why would Nb7 be the worst move?

It's not the worst actually. But knight had 7 legal moves, 4 of there are deadly, 3 of there are save. Then I chose the worst legal save move.