This past Saturday, I watched a few minutes of the third set of a D-I singles match. A beat B 4 in the third. Early in that set, B doublefaulted. A's teammates lining the adjacent court at the indoor facility cheered just as raucously as if A had served an ace. An umpire chaired the match. Qs: 1. Is that common in your experience? 2. Does any Rule prohibit such a practice?