It's always amusing when you can trap your opponent's knight with pawns in the opening. One example is 1.d4 e5? 2.Nf3? e4 3.Ne5? d6 4.Nc4 d5 4.Ne5?? f6. Here's another, similar trap:
I was surprised to see that even though 4...Ne4?? was played thrice in BigBase's 3.5 million games, White only found 5.f3! in one of those games. Perhaps White feared 5...e6, threatening 6...Qh4+, but Black's queen can't mate by herself, and it takes too long to call up the reserves. Simply 6.fxe4 Qh4+ 7.Kd2! wins easily after (a) 7...Qxe4 8.Nf3 Bb4+ 9.Nc3 Nc6 10.Qe2! or (b) 7...g6 8.Nc3 Bh6+ 9.Kc2 Qf2+ 10.Qe2.