Sam Allardyce was left to rue an uncharacteristic mistake from Jussi Jaaskelainen after West Ham surrendered a lead to lose 3-1 at Norwich.
Ravel Morrison's first-half goal had given West Ham a deserved half-time advantage against Chris Hughton's struggling side at Carrow Road on Saturday.
But veteran goalkeeper Jaaskelainen gifted Norwich an equaliser nine minutes into the second half when he dropped the ball and then upended Gary Hooper as he attempted to reclaim it.
Hooper converted the resulting penalty and Norwich went on to secure three precious points courtesy of a fine Robert Snodgrass free-kick and a stoppage-time strike from Leroy Fer.
West Ham had only conceded one goal away from Upton Park in the top flight before the game, and Allardyce cut a frustrated figure after a defeat that saw them drop below Norwich into 16th place - just a point above the relegation zone.
Allardyce told Sky Sports: "We let Norwich back into a game that we shouldn't have let them into. We've only got ourselves to blame.
"What I'm disappointed about is the confidence we've gained from our away results this year was there for all to see in the first half. We'd only conceded one goal away from home and that shouldn't have been a goal.
"We capitulated from our own errors and let Norwich back into a game that we shouldn't have let them into. We've only got ourselves to blame.
"I didn't expect that error from Jussi and the error for the second goal. It was such an unusual error and big error from Jussi, the players looked round and I think it killed them."
|Kenya 0 Ethiopia 0 (agg 0-2): Hosts sent out of CHAN qualifiers after second penalty miss|
|Dortmund put 17 past Olympians, Bremen hand out thrashing|
|Dinamo Zagreb confirm arrests of coach and club president|
|I didn´t know who Wimbledon guest Klopp was - Brown|
|LeBron James backs Liverpool captain Henderson for FIFA 16 cover.|
|Ramos: I am not negotiating with anyone|
|Valencia complete Mina swoop|
|United States v Japan: US seek final redemption|
|Hanousek consigns Celtic to defeat|
|Roma confirm Dzeko interest|
|Arsenal forward Podolski completes Galatasaray switch|
|Warm Dortmund welcome delights Tuchel|
|Silva vows to fight for Madrid place|
|Mancini hints at Shaqiri and Santon departures|
|Bojan raring to go after return from injury|
|Babel backs Firmino to shine at Liverpool|
|Falcao must wait to impress, warns Chelsea favourite|
|Bartomeu: Deal in place for Turan|
|Parejo re-signs with Valencia|
|MLS Review: Sounders return to winning ways|
|Klinsmann unhappy with USA performance|
|High hopes for Paraguay - Diaz|
|Gareca: More to come from Peru|
|Gerrard had European offers|
|Argentina out to spoil Chile´s party - Martino|
|Sampaoli: Chile won´t change style|
|Di Maria will come good at United - Rojo|
|Peru 2 Paraguay 0: Gareca´s men finish third at Copa|
|United States 4 Guatemala 0: Klinsmann´s men win comfortably|
|´I’d prefer no-one to know my name´ - Bassett|
|Matthews completes Sunderland move|
|Podolski set for Galatasaray medical|
|Gundogan inspires Dortmund victory|
|Inter confirm Montoya capture|
|Murillo joins Inter, as Kuzmanovic and Obi depart|
|Falcao signing not a risk, says former Chelsea star|
|Deeney agrees new Watford deal|
|Confidence and service the key to Falcao success at Chelsea|
|No bids for ´Chelsea target´ Baba, claim Augsburg|
|Derby swoop for Ince|
|Fiorentina consider legal action over Salah ´no-show´|
|Chelsea the team to beat in the Premier League, says Neville|
|Storari exits Juve for Cagliari|
|UEFA removes FFP restrictions on Man City and PSG|
|West Brom hope for new owners within the month|
|Chelsea agree loan deal for Falcao|
|Kuyt eyes trophies on Feyenoord return|
|Chile v Argentina: History beckons ahead of Copa America final|
|Wallace extends Monaco loan|
|Mihajlovic wants to instill fear factor at Milan|
|Mignolet improving at Liverpool - Grobbelaar|
|Mumbai City name Anelka player-manager|
|Olivia the parrot predicts Japan to win Women´s World Cup|
|Lusitans coach blasts Bilic´s ´lack of respect´|
|Sakho eyes more Europa League wins|
|Gareca wants Peru to keep building|
|Chile ´no chance´ if Argentina produce their best - Diaz|
|Ocampos joins Marseille permanently|
|Europa League Review: Cloud-nine Brondby ignite opening round|