Lesson
White to move. Main line: 1. d4 Nf6 2. c4 e5 3. dxe5 Ng4 4. Bf4 Nc6 5. Nf3 Bb4+ 6. Nbd2 Qe7 7. a3 Ngxe5 8. axb4 Nd3#
The greedy 7.a3?? invites 7...Ngxe5!, and after 8.axb4 (grabbing the bishop) comes 8...Nd3#. The knight on d3 checks the king on e1, which is smothered by its own pieces on d1, d2, e2, f1 and f2 — mate.
Instead of saving the attacked bishop, Black hurls two more pieces into the fire because the mating net is already there. It is the classic warning against chasing material in a sharp gambit while your own king sits in the center.