Chess opening trap in Caro Kann defence Classical variation

This trap occurs in the Caro Kann defence, Classical Variation, after 1.e4 c6, 2.d4 d5, 3.Nc3 dxe4, 4.Nxe4 Nd7, 5. Qe2 Nfg6??, with 6. Nd6# (note: 12 games featured this trap, in one month on FICS) 5.Qe2 looks awkward and innocuous, but with the Knight on e4, it sets up the potential for a discovered pin on Black’s e7 pawn, leading to the checkmate.