Sam Allardyce was left to rue "one of those days" as his Hammers dominated against Burnley but ultimately lost 2-1 after a smash and grab raid.
After a first half totally dominated by the home team, Kevin Nolan's opportunist strike finally broke the deadlock on 52 minutes. Nolan had missed an open goal in the first 45 and the Hammers looked the likeliest to score again until Burnley's Chris McCann headed in an equaliser against the run of play just before the hour.
Substitute Sam Vokes then broke Hammers' hearts with a 75th-minute header, and when Jack Collison and Carlton Cole hit the post with good efforts it always looked like being a frustrating afternoon.
Big Sam struck a positive tone when reflecting on a day when leaders Southampton also lost, at least keeping the gap on first place to just two points.
"It is a disappointing result for us after a fantastic performance," said the manager. "Unfortunately we haven't got the win we wanted and instead of taking the point as we always talk about, we have thrown it away.
"It is not because we haven't played well, it is not that we haven't dominated, it is about the fact we have switched off on two occasions - the only two dangerous situations they had in the whole game. It was one of those days.
"We have hit the post a couple of times and balls are flashing in front of goal and dropping down and just not dropping to us. It was a shame after such a wonderful goal from Kevin Nolan. His quality was outstanding to get us on our way."
Another low note was Matt Taylor limping off with a calf problem, although it remains to be seen how serious it will prove. The fact that a sixth straight victory was not forthcoming has just left everyone determined to bounce back at Reading next Saturday.
"Really we have no excuses. We shouldn't have lost. We have and we just have to accept it. It was tremendous commitment from the players, just not the right result today unfortunately.
"Sometimes these things happen. It is the first time we have lost a game of football from in front. If they had created lots of chances I could say they deserved the win but they have pinched one. That happens in a long old season, sometimes you don't get what you deserve."