When someone in a crowded room sneezes, and another person sneezes no more than 5 minutes later. This is the essence of the sympathy sneeze. Also, when a loud, spit-flying sneeze is let off in an extremely quiet room, some caring person may choose to fake a sneeze just as loud in order to minimalize embarassment.
Kid: Did you hear Jill sneeze in the middle of the SAT's?
Other Kid: Yeah, I let off a sympathy sneeze, just to be nice.