Chopin omitted a 
lowering b2 to b
2 at the beginning of bar, and a 
restoring b1 in the 2nd half of bar. An oversight of both the alteration and its canceling may seem improbable but happened to Chopin a few times, e.g. in Etude in F minor, op. 25 no. 2, bar 56.
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category imprint: Editorial revisions
issues: EE revisions, Omissions to cancel alteration, GE revisions, Omission of current key accidentals, Errors of A, Errors repeated in FE
notation: Pitch