If he was using an a/d, it could be a number of things. Also, could be the mix itself. Most soundboards when you peak a channel or group will crackle when it clips. If it's something that's not heavy in the mix, you won't notice in the stacks, as it's buried in the SPL of everything else. Very common to hear it in the tape, but not in the mains. He could also be clipping in the JB3, or any number of things.
My JB3, which is normally pretty stable with the V3->C02, was clicking all afternoon yesterday. Only thing different was a new RS/Monster (they use the same packaging) Premium TOSLINK cable. I'm shitcanning the C02 & getting a HOSA 312 this week.
Rick