Diddy made a surprise appearance on "Lopez Tonight" last night -- but the crowd momentarily turned on him when he revealed what he was doing there!

The rapper showed up to announce he'd be hosting a big party to celebrate his new song "Hello Good Morning" going gold on George Lopez's show next week ... when a different audience would be in attendance.

The crowd wasn't too happy with the news -- but Diddy found an awesome way to make it up to them.

He'll perform with Diddy-Dirty Money on the show next Wednesday, July 28.

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