The first time he did it was for writing the screenplay to his breakout hit, Pulp Fiction, and it turns out that Steven Spielberg predicted to Tarantino that he would win, before there was even a nomination.
Tarantino recently appeared on The Howard Stern Show and talked about his friendship with Steven Spielberg.
The one award that Pulp Fiction did win was for Best Original Screenplay, an award Forrest Gump was not up for, because the movie won the award that night for Best Adapted Screenplay.
Tarantino says that Spielberg prides himself on knowing the business, and here he certainly showed that he did.