.... and this purely hypothetical idiot asks you for advice with the
following problem:

He (hypothetically, of course) puts some pencil marks on the wrong
edge of a face frame, the outside edge. He realizes his mistake and
redoes the marks in the proper place, intending to sand off the ones
he made in error. For the sake of argument, let's suppose he forgets
to do so, partly because he thought that edge would butt up against
another unit and partly due to general absent-mindedness.

Our fictional novice woodworker then proceeds to finish over the
marks, only discovering that they will indeed be on an exposed side as
he applying the tenth clamp to glue the face frame to the box. He does
not take the whole works apart at this point.

So, were such an improbably boneheaded error to actually happen, how
would you advise him to fix it? I imagine he would first think to sand
it with some pretty coarse sandpaper to get through the poly and
pencil, then smooth and refinish. But that sounds pretty laborious. I
wonder if he could shave off the tiniest amount with a router (our
hypothetical face frame is maybe 3/32" proud of the cabinet edge) and
straight-edge.