There is a song I heard quite some time ago. It is by a classical composer, and I think it was Mozart. Somewhere in the middle, the cello plays a brief riff with the same famous chord progression as "Heart and Soul", I, vi, IV V. I think it repeats it twice. The rhythm is quarter, eighth-eight, and each chord is (semi-) arpeggiated starting on the root, going up to the third, and then back to the root (and then the next chord).
Does anyone know what song this is? I remember hearing it when I was young and being startled to hear a "pop" riff in a classical piece. Part of what was striking about it was that it wasn't really integrated into the piece --it was played as an isolated line, just like a pop breakdown.
Here is the riff in an online sequencer --it may be easier to understand it than through the text description.