A Martin Dubravka error saw Newcastle United suffer an eighth successive defeat at Wembley.
The goalkeeper fumbled a late shot from Heung-Min Son this afternoon.
And the mistake saw Rafa Benitez's spirited side beaten 1-0 by Tottenham Hotspur.
The game was watched by owner Mike Ashley, who had sanctioned a club-record £21million move for Miguel Almiron on transfer deadline day.
Newcastle remain 14th in the Premier League.
Ashley was at an icy and half-empty Wembley, who lumps of snow were falling from arch on to the pitch.
Benitez – whose side was backed by 3,200 fans – fielded an unchanged side and named new signing Antonio Barreca on the bench.
Tottenham dominated possession in the first half, but they found opportunities harder to come by against a disciplined and organised Newcastle side.
Lucas Moura should have done better with a 17th-minute header – he put it wide from seven yards after Erik Lamela clipped the ball back in to him – and United were looking comfortable.
Lamela struck the woodwork with a header of his own midway through the half.
United's first real chance came at the half-hour mark.
Matt Ritchie broke up the left, and his ball was helped on by Christian Atsu. Ayoze Perez and Salomon Rondon both had shots blocked by Tottenham, whose fans were becoming increasingly uneasy as the half wore on.
Lumps of ice continued to fall from the arch during the game.
Martin Dubravka parried a shot from former Newcastle midfielder Moussa Sissoko late in the half and the goalkeeper's team-mates cleared the danger.
Davinson Sanchez wasn't far away with a chance for Tottenham early in the seconde half.
Rondon, however, was even closer in the 51st minute. The striker played the ball out to DeAndre Yedlin on the right and continued his run to the box, where he met a cross with his head. His shot struck the post.
Tottenham, with 30 minutes left, pushed for a goal, and United had Fabian Schar to thank for a superb goalline clearance.
Schar hooked an effort from Christian Eriksen off the line. Perez soon tested Hugo Lloris with a shot at the other end of the pitch.
Newcastle were dogged and determined without the ball, and it looked like they would, at the very least, see the game out.
However, Dubravka fumbled Son's dipping shot in the 83rd minute.
There was no way back for United at Wembley, where the travelling fans got their first glimpse of Barreca in the final minutes of the game.
NEWCASTLE UNITED: Dubravka; Yedlin, Schar, Lascelles, Lejeune, Ritchie, (Barreca, 86); Perez, Longstaff, Hayden, Atsu (Kenedy, 82); Rondon. Subs not used: Woodman, Manquillo, Manquillo, Fernandez, Clark, Joselu.
TOTTENHAM HOTSPUR: Lloris, Trippier, Alderweireld, Sanchez, Vertonghen, Winks, Sissoko, Eriksen, Lamela (Rose, 78), Lucas (Llorente, 60), Son (Dier, 89). Subs not used: Gazzaniga, Foyth, Walker-Peters, Wanyama.
Goal: Son 83
Booking: Yedlin 65
Referee: Andre Marriner (West Midlands)