Furious Roberto Mancini hauled off Mario Balotelli for an outrageous bit of showboating.
The Italian bad boy had already put City ahead with a cheekily taken penalty.
But he then wasted an easy chance to double the visitors' lead, as he was sent clean through by David Silva on the half-hour.
Instead of applying a simple finish, he tried an extravagant pirouette and backheel and saw his weak effort dribble wide of a post.
It drew loud boos from the home crowd and clearly annoyed Mancini, who immediately told sub James Milner to take over.
The pair exchanged angry words when Balotelli made his way off, with the 20-year-old apparently claiming he thought he was offside.
Mancini said: "I hope this is a lesson for him
In football you always need to be professional, always serious, and he wasn't.
"He needs to understand his behaviour has to be good in every game, not just in a semi-final or final."
Overall, it suggests this could be another eventful season for the former Inter Milan star, who joined City for £23million last summer and regularly found himself in bother.
Yet early in the game he had looked in the mood to make some headlines of the right kind as City got off to a bright start.
Micah Richards was felled by Bryan Jordan in the box and Balotelli sent the keeper the wrong way from the spot. Then he let himself down again, with local commentators suggesting he had lacked respect for Galaxy and the event.
David Beckham had been subbed by the time Galaxy levelled with a dipping volley from sub Mike Magee early in the second half.
England's goalkeeper Joe Hart saved two spot-kicks in the shoot-out to decide the match before blasting in the winner himself as City edged it 7-6.
The Italian bad boy had already put City ahead with a cheekily taken penalty.
But he then wasted an easy chance to double the visitors' lead, as he was sent clean through by David Silva on the half-hour.
Instead of applying a simple finish, he tried an extravagant pirouette and backheel and saw his weak effort dribble wide of a post.
It drew loud boos from the home crowd and clearly annoyed Mancini, who immediately told sub James Milner to take over.
The pair exchanged angry words when Balotelli made his way off, with the 20-year-old apparently claiming he thought he was offside.
Mancini said: "I hope this is a lesson for him
In football you always need to be professional, always serious, and he wasn't.
"He needs to understand his behaviour has to be good in every game, not just in a semi-final or final."
Overall, it suggests this could be another eventful season for the former Inter Milan star, who joined City for £23million last summer and regularly found himself in bother.
Yet early in the game he had looked in the mood to make some headlines of the right kind as City got off to a bright start.
Micah Richards was felled by Bryan Jordan in the box and Balotelli sent the keeper the wrong way from the spot. Then he let himself down again, with local commentators suggesting he had lacked respect for Galaxy and the event.
David Beckham had been subbed by the time Galaxy levelled with a dipping volley from sub Mike Magee early in the second half.
England's goalkeeper Joe Hart saved two spot-kicks in the shoot-out to decide the match before blasting in the winner himself as City edged it 7-6.
LA Galaxy-Manchester City 6-7 (1-1, goals: Balotelli, Magee)
