David Moyes was frustrated to see Sunderland concede another last-ditch goal but believes it is down to the weight of pressure the side has come under.
Sunderland were heading for a first clean sheet of the season when they switched off at a corner in the fourth minute of stoppage time.
It was to prove costly as the flag kick, taken short by Dimitri Payet and Mark Noble, led to Winston Reid shooting home from 20 yards.
There was controversy with the Black Cats claiming the goal should have been disallowed for offside, arguing that there was a Hammer interfering with play on the edge of the six-yard box.
Whatever the rights and wrongs, it was the fourth time they have leaked goals in the last five minutes after the games against Man City, Southampton and Crystal Palace.
The five points coughed up from those games would have seen Sunderland on the edge of the relegation zone intsead of rock bottom.
"I think it comes down to the pressure we've had to take in games," explained Moyes.
"Eventually, if you are accepting the opposition coming on to you it can affect you in the end.
"If you keep giving the ball away too, we did that three or four times in the second half, that makes you more liable to concede."
But Moyes believes Sunderland will glean positives from their first visit to the London Stadium.
"I think we can take confidence from the game, I have to say the players performed well," said the Scot.
A draw looked unlikely in te opening stages when West Ham bossed the game, with Payet hitting the post, as well as bringing a save from Jordan Pickford.
Moyes was pleased with the resilience shown and, in the end, the second half was even. A goalless draw would have proven a fair result.
"Having watched the first half an hour there looked like being only one outcome and West Ham would win it," said the boss.
"It was very difficult, dealing with Payet, Lanzini and their formation.
"It took us a while, but we got to grips with it and grew into the game."