“Just this once” is how they get you to twice.

The one-time exception is rarely meant as one. It tests whether the rule moves, and once it moves, the next ask arrives with the precedent already built in: you did it before, so what is different now. The person asking may not be planning any of that. Patterns form on their own around whatever worked last time.

When you hear just this once, decide by asking whether you would want it as the new normal. If yes, fine, go ahead. If no, here is the cheapest place to hold the line, because the second ask is much harder to turn down than the first.