If an accidental is omitted from a different octave position on the staff, it is not an exception to the rule but an editorial error. The rule has no exceptions.
haha that's a fun name, "courtesy accidentals".I can't find an example, but I also feel like there are some exceptions to the rule sometime, where an explicit accidental is omitted for an octave up/down, but it sounds weird if you don't play it, so it seems to be implied. So in the example I gave with the middle C, in some cases it's actually expected/implied that the subsequent Cs (octave up/down) should also have the accidental applied to them. Am I wrong on this, or does this actually happen?Another small question: suppose middle C is sharped, in treble-clef (e.g. right hand) and in the same bar, the left hand in bass-clef later has to play the same middle C. I assume it should still retain the accidental in such cases?
In modern editions, accidentals are never implied to apply to a different octave than the one they were written in front of. Can you look for/try to remember an example? I'm quite curious where you have seen this.