Everyone knows 2 knights cannot deliver mate but whilst this is true a co-operative Black can manage to lose!
1.Nf8
Kg8
2.Nd7
Kh8
3.Nd6
Kg8
4.Nf6+
Kf8
and there is no way of forcing mate [4...Kh8
but this blunder would allow mate 5.Nf7#
] *