This effect pops up a lot in commercials; there's a Claritin ad
running now that does this. A guy's sneezing as the backdrop changes
behind him, showing all of the outdoor fun he's missing because of his
allergies.
It also shows up - somewhat different context - in the movie
Casablanca. Rick and Ilsa are in the car, in the Paris flashback, and
the background dissolves behind them while their conversation
continues uninterrupted. It's a VERY stylized process shot, all
things considered.
--
Life Continues, Despite
Evidence to the Contrary
Steven