In this puzzle,
http://en.lichess.org/training/46017
only Qa7+ is accepted, but there is an equally fast mate from first move Qc7+
1. 79 [M5] 40.Qc7+ Ka4 41.Ra3+ Kxa3 42.Qa5+ Na4 43.Qb4+ Kxa2 44.Qb3#
2. 79 [M5] 40.Qa7+ Ba6 41.Ra3+ Na4 42.Rxa4+ bxa4 43.Qc7+ Bb6 44.Qxb6#
http://en.lichess.org/training/46017
only Qa7+ is accepted, but there is an equally fast mate from first move Qc7+
1. 79 [M5] 40.Qc7+ Ka4 41.Ra3+ Kxa3 42.Qa5+ Na4 43.Qb4+ Kxa2 44.Qb3#
2. 79 [M5] 40.Qa7+ Ba6 41.Ra3+ Na4 42.Rxa4+ bxa4 43.Qc7+ Bb6 44.Qxb6#