Fewest moves from start to tactic

[Marek Soszynski]

Years ago, when I wanted to quickly test an engine's tactical speed, I used to input 1.e4 e5 2.Nc3 Nc6 3.Bc4 Na5??. Of course this is far too easy nowadays–and was never that challenging anyway.

What, in your opinion, is the most taxing tactic reached in as few moves as possible from the regular starting position?