I'll post my thoughts on what I think your ADR meant and maybe others will chime in with their interpretation:
What I think your ADR meant was that this is a mistake and the supervisor has to be given the opportunity to fix the mistake before you file, not that you can't ever grieve this.
For example, you point out the mistake to your supervisor, s/he says, "Oops! Sorry. My bad. I'll get it cleared up for you." And they do, in fact, correct the mistake; therefore, no grievance
Or, you point out the mistake, the supervisor says, "No, I don't think I made a mistake. You were paid correctly." Then you have a grievance.
I just spent about four months---but it seemed like years!--with a supervisor who screwed up payroll in an office every pay period. I am not exaggerating. Every pay period. He would always apologize, agree to fix the errors, and then he would not do it. Over and over and over again. So you can experience that, as well. If they don't correct the error, you have a grievance, but you have to give them the opportunity to fix it.