I began working on Chopin's Etude Op 10 No 9, and all was going well until I got to measure 22 and hit A#/Bb0 and F#/Gb5 together. Ouch. Depending on your key identification system, the two keys are also referred to as Bb1 and Gb5.
Basic users, give it a try and see if you hear a strong dissonance/discord between these two keys. To me it's very noticeable either through headphones or my speaker system. It's especially noticeable when played in the context and flow of the etude and sounds as if the piano is quite out of tune.
I just played the etude using Pianissimo and everything is dead-on including the notes mentioned when played together. Beautiful.