In Bar 21 of Chopin's waltz in A minor the 8th note in the right hand is an interesting case in that it is a G at the start of an octave higher section which is clearly intended to be G#a G♯, as the previous occurrence of the note at that position in the stave but not an octave higher - and all the other Gs in the bar - are sharp, but arguably it should have an accidental as it is not the same G.