This one puzzled me for a while- I found the line quickly, but it took me a while to understand why it was the shortest.
1br3r1/1Bp5/1pN1pB2/1P1RPPp1/2PPk1P1/K3Pp2/2b2P2/4N3 w - - 0 1
I was solving this puzzle during one hour and I managed to get mate in 6. After that I decided to look at the position again. Suddenly I saw that I had forgotten to put the white rook on d5. As soon as I put it in place I solved this puzzle. Actually it was very easy.
1.Rc5! Bb12.Na5+ c63.Nb3 Rh84.Nd2#
1.Rc5 Ba72.Nb4+ c63.Na2 Rh84.Nc3#
Rc5 is a key move because it protects the king from check.