Hi I just asked the question about sharps, flats and naturals. I guess the altered note in one clef but not the other would never really come up because you'd just change the scale degree.. like if you had a C7#9 or whatever you'd never write the #9 note as Eb, you'd write it as D# so that way you don't have an E and Eb)...
that's true but there are some situations when it gets really dense when you might need F and F# at the same time. let's say if you are scoring a horror movie or something you might even need them in the same octave because you have F, F#, G all mashed together. i honestly have no idea how you would notate that. it is rare and only in extremely dissonant 'horror' type situations