Had it happen 3 or so weeks ago. Late in the game and the team that's trailing scores - bringing them within a touchdown. We configure our officials for an expected on-side kick and sure enough, it happens. When an R player falls on the ball, the official on R's restraining line closest to the ball blows his whistle, but the ball is now skittering back towards K's restraining line. R never had possession. K ended up with the ball but our only course of action was a re-kick. In the ensuing re-kick, R was ready for the on-side kick and recovered it. You hate to have that kind of situation where you might have changed the outcome (regardless of weak the odds were that K could have subsequently scored the go-ahead points in the time left).