The decision to start with Mario Balotelli, Carlos Tevez and Edin Dzeko in the same XI for the first time failed just as spectacularly as the champions failed to score for the first time in almost seven months.
It could have been even worse for Mancini's men had Kevin Nolan not seen an early goal controversially ruled out for offside as West Ham more than held their own and were marshalled brilliantly by Winston Reid and James Tomkins.
There were no excuses for the shocking city defending that should have seen them behind inside four minutes. Mark Noble played a simple straight free-kick over the top, which Nolan brilliantly volleyed first time past Joe Hart but the assistants' flag was up instantly, although replays showed Nolan may have been level.
Five minutes later and City should have been bemoaning an offside flag but Dzeko headed a simple rebound wide anyway after Jussi Jaaskelainen parried Samir Nasri's stinger.
Jaaskelainen almost fumbled Tevez's snapshot after the restart before getting away with spilling the striker's corner as City threatened but West Ham got going again and Nolan failed to glance Yossi Benayoun's cross on target before Andy Carroll did likewise with his own overhead attempt.
That was nothing compared to one of the misses of the season midway through the half fromGareth Barry, who controlled Tevez's ball on his chest but somehow prodded wide from barely three yards.
Suddenly, City were swarming but Reid and Tomkins were producing heroics at the back.
West Ham were also a threat on the break, Carroll's lunging finish after more fine work from Jarvis his final act before the introduction of Carlton Cole.
Gary O'Neil and George McCartney were both booked and Sergio Aguero almost caught out Jaaskelainen from a quick free-kick.
Source: PA
Source: PA