You have to go back to 1968 to the last time this happened. On March 12 of that year, Lyndon Johnson barely beat Eugene McCarthy by 49-42% in the NH primary. This poor showing by Johnson encouraged Robert Kennedy to throw his hat in the ring and shortly afterwards, Johnson, sensing his doom, dropped out of the race.
You forgot about Bush vs Buchanan in NH during 1992. At least McCarthy was a senator. Buchanan was a political columnist, and he beat Bob Dole in NH in 1996.