A includes a cautionary 
before the 11th semiquaver, however, in FE both accidentals were printed – and 
. At the same time it is not certain which of these accidentals was printed first and which is binding. Therefore, there are two possibilities:
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The natural was replaced with a flat. It would mean that Chopin wanted to change d to d. This option fits well into the adopted scheme of correlation between the sources – the drawn from the manuscript is changed to a in the last proofreading. The change was not included in GE, which is based on the copy of FE which does not feature the latest corrections, yet it is in EE, which has finished FE as the base text.
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The flat was replaced with a natural. This possibility requires to assume that first the engraver of FE committed a mistake by inserting a instead of a , which Chopin then corrected and the engraver of EE misinterpreted the outcome of the correction. The accumulation of two errors is not impossible in this case – the described ambiguity of FE may have easily confused the engraver of EE, while the mistake of the engraver of FE could have been caused by the presence of a several dozen of flats in this and the previous bars and a striking similarity of the upper part of the in A to a flat. An additional argument is the fact that a natural cannot entirely "cover" a flat, while a flat is not able to cover the lower part of a natural. Being conscious of that, the engraver would have probably not taken the decision of a simplified correction (without removing the original accidental) if he had had to insert a flat instead of a natural (cf. the Etude in F major, No. 8, bar 79).
According to us, the second presented possibility is more likely. An additional argument for adopting the version with d in the main text is its undeniable authenticity and analogy with bar 66.
Compare the passage in the sources »
category imprint: Graphic ambiguousness; Differences between sources
issues: Errors in FE, Errors in EE, Cautionary accidentals, Authentic corrections of FE
notation: Pitch