What is the "fruit roll" and when was it not successful?

From the text:

But you could depend on the substitute to be mildly retarded, so I'd pick out the right moment and clear my throat quite loudly-which was the signal for everyone to get the apples out. Then I gave this phony sneeze that meant to hold them down near the floor. When I whistled, that was the signal to roll 'em. Did you ever hear a herd of buffalo stampeding? Thirtyfour scrawny, undernourished apples rolling up the aisles sound just like a herd of buffalo stampeding.

Every one of the fruit rolls was successful, except for the time we had a retired postman for General Science 1H5. We were supposed to study incandescent lamps, but he spent the period telling us about commemorative stamps. He was so enthusiastic about the old days at the P.O. I just didn't have the heart to give the signals, and the kids were a little put out because they all got stuck with old apples.