I echo @Pondlife's answer - base your response on the severity, and if in any doubt loop in AOPA legal. If you do nothing else, file a NASA ASRS form; it's not a guarantee but it can do no harm.
As a student pilot 15 years ago, I got The Number during an early solo at Paine Field (KPAE) near Seattle.
I received taxi instructions to 34L at A4, did my runup, and called Tower requesting closed pattern, 34L. I was cleared for takeoff, runway 34R. On the upwind, Tower asked me to switch to 132.95... the west-side Tower frequency. Oops. Then they asked me to write down The Number. I finished my landings, because you gotta get back on the horse, then landed, told my instructor, and we called.
By that time the manager had pulled the tapes and determined that, yes, I missed the runway identifier, but the controller also didn't catch that I said I was at a 34L intersection. Everybody was both at fault and not really at fault. Apparently another plane on long final went around, but it wasn't anything crazy dangerous.
All that resulted from the incident was, about two weeks later, the appearance of signs reading "TOWER 132.95" and "TOWER 120.2" at the appropriate intersections. They're still there today! :)