Thanks all. A bunch of us had this discussion after I had play number 1 in a game. As usual, that discussion branched out to the others.
In my game, I denied the substitution because time had not run off the clock (thinking of 3-3-4 but using it in the wrong situation). It felt right at the time and the coach, who is also an official in our association, agreed after we talked about it for a few seconds.
But after the game, it just didn't make sense to me - and now we all know why!